Th`ese

pr´esent´ee pour obtenir le titre de

Docteur de l’Universit´eLouisPasteurdeStrasbourg sp´ecialit´e:Astrophysique

par H´el`ene Roussel

— · — Emission en infrarouge moyen des poussi`eres dans les galaxies spirales : liens avec la formation d’´etoiles et avec la dynamique des galaxies barr´ees — · —

jury compos´ede: Mme Evangelia Athanassoula rapporteurs Mr Olivier Bienaym´e Mr George Helou Mme Agn`es Acker pr´esidente du jury Mr Laurent Vigroux directeurs de th`ese Mr Alain Blanchard

laboratoire d’accueil : Service d’Astrophysique du CEA C.E. Saclay 91191 Gif sur Yvette cedex

Depuis le fameux big bang C¸a tourneca ¸ tourne De l’atomeal’´ ` etoile Alafa¸con d’un big band C¸a tourneca ¸ tourne C¸ a tourne au quart de poil [...] Au dancing des galaxies C¸a tourneca ¸ tourne Oh!labellebouledebal! Il n’y a gu`ere qu’ici Qu’¸ca tourne,ca ¸ tourne C¸ a tourne toujours mal [...]

Claude Nougaro

Merci `a Laurent Vigroux de m’avoir judicieusement fait confiance pour cette th`ese, en me donnant acc`es `a un ensemble de donn´ees d’un int´erˆet inestimable (grˆace au travail de pr´eparation de toute l’´equipe ISOCAM) et, malgr´e ses multiples occupations, d’avoir m´enag´e une petite par- tie de son temps pour me guider et m’aider. Marc Sauvage m´erite ´egalement ma reconnaissance pour avoir ´et´e un co-directeur de th`ese enthousiaste et tr`es p´edagogue. Merci aux membres du jury d’avoir manifest´e de l’int´erˆet pour ma th`ese et d’avoir ´et´eau rendez-vous de la soutenance, malgr´e les emplois du temps tr`es serr´es. Rachida Sadat et Alain Blanchard m’ont les premiers donn´eunavant-goˆut du travail de recherche. Je leur suis redevable de m’avoir pouss´ee `a publier les r´esultats de stage et d’y avoir grandement contribu´e, en continuanta ` travailler avec moi au cours de la th`ese. J’ai aussi b´en´efici´e de l’attention qu’Albert Bosma a port´ee `a mon travail, de sa connaissance approfondie des galaxies et de ses marques d’encouragement. Que Doris Neumann, Ya¨el Fuchs et ses l´egendaires Strudels, le transfuge Thomas Douvion, Elena Belsole, Jean-Paul Mbelek, Pascal Gallais, Fran¸coise Gaulier, Lydie Koch-Miramond, Suzanne Madden, David Landriu, qui m’a accueillie au d´ebut de la th`ese, et d’autres, soient remerci´es de leur amiti´e et de la sympathique atmosph`ere qui les entoure, qui rend le travail tellement plus agr´eable. Je n’oublie pas nos secr´etaires et biblioth´equaire d´evou´ees, Yolande Guillaume, Genevi`eve Thierry, Elisabeth Rainot et Dominique Monvoisin. Je garderai aussi un souvenir ´emu du personnel de s´ecurit´e du CEA et des suaves annonces par haut-parleur tombant d´elicatement du ciel...

Table des mati`eres

I Introduction 1

II La poussi`ere dans les galaxies 3 II.1 Rˆoles de la phase poussi´ereuse 3 II.2 Signatures observationnelles 5 II.2.1 Absorption et rougissement 5 II.2.2 Rayonnement infrarouge 7 II.3 Formation et destruction des grains de poussi`ere 11 II.4 Apports d’IRAS `al’´etude des galaxies spirales normales 12 II.5 Centres galactiques observ´es en infrarouge 15

III Les galaxies spirales barr´ees 17 III.1 Un bref historique 17 III.2 Aspects morphologiques 20 III.3 Dynamique du gaz 23 III.4 Simulations des barres incluant le gaz et la formation d’´etoiles 26 III.5 Importance des barres pour l’´evolution des galaxies isol´ees 28

IV Echantillon de galaxies 31 IV.1 Programmes d’observation 31 IV.2 Publication : Atlas 32 IV.3 Suppl´ement `a l’Atlas publi´e : les galaxies des programmes Sf glx et Irgal 73

V Formation d’´etoiles et rayonnement de la poussi`ere 101 V.1 Morphologie compar´ee en infrarouge moyen et en Hα 101 V.2 Dans les disques 115 V.2.1 Publication 116 V.3 Pr´ecisions sur les cons´equences pour l’infrarouge lointain 133 V.4 Dans les r´egions circumnucl´eaires 135

VI Activit´edeformationd’´etoiles au centre des galaxies barr´ees 145 VI.1 Publication 145 VI.2 Taille et brillance des r´egions centrales 169

VII Conclusions et poursuite envisageable 175

A Traitement des donn´ees et photom´etrie 179

B Publication : Composante baryonique des amas de galaxies 195

C Liste de publications 219

1

Chapitre I Introduction

Le rayonnement infrarouge des galaxies constitue une part essentielle de leur bilan ´energ´etique, contribuant pour environ un tiers dans les galaxies normales, et dominant la puissance bolom´etrique dans certaines galaxies dont l’activit´e de formation stellaire, ou li´ee `a un noyau de Seyfert, est suraliment´ee par des interactions violentes avec d’autres galaxies. Ce fait est suffisant `amotiver l’int´erˆet port´e aux sondages de galaxies r´ealis´es en infrarouge, car les populations s´electionn´ees sont diff´erentes de celles s´electionn´ees par leur ´emission stellaire.

Le contenu en poussi`ere des galaxies est en principe fortement coupl´e`aleur´evolution chim- ique. La poussi`ere existe n´eanmoins en abondance mˆeme dans des galaxies tr`es sous-m´etalliques (Thuan et al. 1999). Les grains se forment dans les ´ejections de masse des ´etoiles ´evolu´ees et croissent dans les nuages mol´eculaires o`u se condensent les nouvelles ´etoiles. Du fait que les grains de poussi`ere absorbent pr´ef´erentiellement les photons de plus courtes longueurs d’onde, ils sont chauff´es efficacement par les ´etoiles les plus massives du voisinage. C’est ce qui per- met d’utiliser l’´emission infrarouge comme indicateur de formation d’´etoiles, en tirant profit du ph´enom`ene d’extinction au lieu d’avoira ` corriger ses effets sur l’´emission stellaire et n´ebulaire. Cependant, en tant que traceur indirect, sa validit´edansdiff´erents types de galaxies ne va pas de soi et doit ˆetre examin´ee soigneusement. Si, dans le cas d’un sursaut de formation d’´etoiles g´en´eralis´e`a l’ensemble de la galaxie, l’int´erˆet d’estimer le taux de formation stellaire par le rayonnement infrarouge fait peu de doute, le cas des galaxies “normales” (ne montrant pas d’activit´etr`es intense, stellaire ou de type Seyfert) est plus probl´ematique. C’est l’un des objets de cette th`ese d’´etudier `a nouveau la question avec, relativement aux observations du satellite IRAS, l’avantage de la r´esolution angulaire et de l’information spectroscopique. Les observa- tions analys´ees ici, r´ealis´ees par la cam´era ISOCAM, couvrent l’infrarouge moyen de 5a18 ` µm, un domaine spectral habituellement associ´e en grande partie aux cirrus dans les galaxies spi- rales normales, c’estadire` ` a des nuages interstellaires diffus chauff´es par un rayonnement peu ´energ´etique, caract´eristique de populations stellaires ´evolu´ees. Nous allons voir comment les nouvelles observations modifient cette vue.

Les galaxies barr´ees, dont c’est l’un des objectifs de cette th`ese d’examiner l’activit´edefor- mation d’´etoiles, ont suscit´e beaucoup d’int´erˆet ces derni`eres ann´ees pour plusieurs raisons. Les simulations num´eriques et les observations ont montr´e qu’une perturbation barr´ee se d´eveloppe tr`es facilement dans les disques galactiques et que la plupart des galaxies spirales de l’univers proche sont effectivement barr´ees ; les galaxies barr´ees ne peuvent donc plus ˆetre consid´er´ees seulement comme une int´eressante curiosit´edynamique.Lar´eponse du milieu interstellairea ` 2 un potentiel gravitationnel barr´e induit une ´evolution lente mais irr´eversible, car le gaz tend `a couler vers le centre. Cela fait qu’une barre est un facteur d’´evolution important pour les galaxies isol´ees ou n’interagissant pas fortement avec leur environnement. L’int´erˆet d’observer leur activit´e de formation d’´etoiles par le biais du rayonnement de la poussi`ere est double : nous ´evitons les effets d’extinction dans les zones de forte densit´e de gaz et de poussi`ere (en partic- ulier dans les zones de choc de la barre, et dans les r´egions circumnucl´eaires), et nous pouvons faire progresser notre compr´ehension de l’excitation des grains de poussi`ere dans les galaxies, de fa¸con g´en´erale. De pr´ecieuses informations ont ´et´eapport´ees par des observations au sol des r´egions centrales de galaxies barr´ees, mais n´ecessairement limit´ees `a des sources tr`es brillantes (Telesco et al. 1993). Par ailleurs, les observations d’IRAS, qui a r´ealis´e un relev´e complet du ciel dans quatre bandes infrarouges, ne permettaient pas d’obtenir une information spatiale sauf dans les galaxies les plus proches. Nous avons ici la possibilit´e d’examiner la morphologie de l’´emission en infrarouge moyen d’un grand nombre de galaxies, de s´eparer spatialement des zones o`u dominent diff´erentes phases de poussi`ere et diff´erents r´egimes d’excitation, et ainsi d’obtenir plus de d´etails concernant les effets de la dynamique barr´ee sur la distribution et l’intensit´ede la formation d’´etoiles.

Cette th`ese comporte deux parties introductives, l’une sur la composante de poussi`ere des galaxies et l’autre sur les aspects morphologiques et dynamiques des galaxies barr´ees, une partie pr´esentant l’´echantillon analys´e et les cartes en infrouge moyen, une partie sur l’estimation des taux de formation d’´etoiles reposant sur l’´emission de la poussi`ere, et une partie sur la comparaison de l’activit´e des galaxies barr´ees et non barr´ees, d´etaillant les propri´et´es des r´egions circumnucl´eaires. Les publications sont incluses `a l’int´erieur des chapitres dans la suite logique du discours, et suivies de r´esultats suppl´ementaires. 3

Chapitre II La poussi`ere dans les galaxies

II.1 Rˆoles de la phase poussi´ereuse

La poussi`ere ne constitue qu’une tr`es petite fraction de la masse visible des galaxies : environ un centi`eme de la masse gazeuse atomique et mol´eculaire, elle-mˆeme de l’ordre d’un dixi`eme de la masse stellaire pour les galaxies spirales. Nous n’allons ´etudier qu’une partie de la population de poussi`eres, celle qui rayonne dans l’infrarouge moyen entre 5 et 18 µm. Dans l’´echantillon ´etudi´e ici, on peut estimer que la puissance ´emise dans la bande 12–18 µm est de l’ordre de 2`a 6% de la puissance infrarouge totale ´emise entre 8 et 1000 µm (en utilisant pour estimer cette derni`ere quantit´e les flux IRAS et la formule de calcul de Sanders & Mirabel 1996). Pour la bande 5–8.5 µm, cette fraction varie de 2a ` 16%. Cependant, les observations en infrarouge moyen permettent de sonder des phases de la poussi`ere diff´erentes de celles qui rayonnent dans l’infrarouge lointain et submillim´etrique (qui constituent l’essentiel de la masse poussi´ereuse), comme nous le verrons plus loin. Malgr´e son caract`ere n´egligeable pour la dynamique des galaxies, la poussi`ere est une composante galactique essentielle et de grand int´erˆet pour plusieurs raisons :

— Elle absorbe et diffuse une partie du rayonnement des ´etoiles et du gaz chaud situ´es en arri`ere-plan, de mani`ere s´elective en fonction de la longueur d’onde. Les photons de plus grande ´energie ´etant pr´ef´erentiellement absorb´es, la distribution spectrale d’´energie ob- serv´ee se trouve modifi´ee de telle sorte que toutes les couleurs sont rougies. L’extinction moyenne le long de la ligne de vis´ee des r´egions HII des disques galactiques est ´elev´ee. Par exemple, van der Hulst et al. (1988) a estim´e l’extinction en direction d’une quarantaine de r´egions HII g´eantes de M51 en comparant les flux de la raie de recombinaison Hα avec une estimation de la composante thermique des flux radioa ` 6 cm (rayonnement de freinage des ´electrons libres). Malgr´e les incertitudes substantielles duesalas´ ` eparation des rayon- nements thermique et synchrotron, les r´esultats indiquent de fa¸con concluante une extinc- tion moyenne en bande V de 1.8 mag, contre environ 1.3 mag par les d´ecr´ements de Balmer (qui tendenta ` sous-estimer l’extinction). L’extinction est de plus localement fortement inhomog`ene, d´ependant de la quantit´e de poussi`ere, de sa composition et de sa distribu- tion g´eom´etrique, toujours complexe et difficileamod´ ` eliser, par rapport aux sources qu’elle att´enue. La poussi`eresetrouveenabondancel`ao`u le gaz est ´egalement le plus dense, dans les nuages mol´eculaires qui apparaissent en optique comme des n´ebulosit´es obscures ; ces nuages ont une structure extrˆemement complexe, fractale sur de nombreux ordres d’´echelle 4

(Falgarone et al. 1991), ce qui implique une forte variation spatiale de l’extinction. Du fait de l’existence de la poussi`ere, notre vue des galaxies est donc d´eform´ee en optique et encore plus dans l’ultraviolet lointain qui est une signature directe des ´etoiles jeunes massives. Les grains de poussi`ere dont la taille est sup´erieure `a la longueur d’onde de la lumi`ere incidente en diffusent ´egalement une partie, et c’est ce ph´enom`ene qui est `a l’origine des n´ebuleuses par r´eflexion.

— Comme la poussi`ere est intimement mˆel´ee au gaz interstellaire, qui est la composante dissi- pative des galaxies – c’est `a dire capable, au contraire des ´etoiles, d’´evacuer par rayonnement son ´energie cin´etique–lapoussi`ere ressent de la mˆeme mani`ere les ondes de densit´e(spi- rales ou barres) et la compression cons´ecutive aux chocs `a grande ´echelle form´es au passage de ces ondes. Les zones de densit´e´elev´ee de gaz mol´eculaire le long des bras spiraux et des barres sont ainsi signal´ees par des bandes de poussi`ere provoquant une absorption tr`es contrast´ee dans les images optiques. Ces bandes de poussi`ere fournissent donc un pr´ecieux traceur indirect de la dynamique des galaxies spirales.

— Des grains de forme allong´ee, en pr´esence d’un champ magn´etique r´egulier, s’alignent dans une direction privil´egi´ee et induisent une polarisation lin´eaire de la lumi`ere, parce que les ondes polaris´ees parall`element `a l’orientation des grains sont plus ´eteintes que celles polaris´ees perpendiculairement. Ainsi,a ` cause de la poussi`ere, observer une direction de polarisation dominante permet de remonter aux lignes de champ magn´etique, qui dans les cas o`u elles sont gel´ees dans le gaz, donnent des indications sur les flots de gaz et la dynamique interne des galaxies.

—L’´energie absorb´ee est r´e´emise dans l’infrarouge `a des longueurs d’onde comprises entre 3 µm et 1 mm selon la nature, la taille et la temp´erature des grains de poussi`ere. Pour des galaxies spirales normales comme celles de l’´echantillon analys´eici,nepr´esentant ni un taux de formation d’´etoiles tr`es ´elev´e ni un noyau de Seyfert tr`es actif, la luminosit´e infrarouge totale est de l’ordre d’un tiers de la luminosit´e stellaire (Soifer & Neugebauer 1991). C’est donc une composante importante du bilan ´energ´etique des galaxies, et ce d’autant plus que l’activit´e, stellaire ou non, augmente : pour les galaxies lumineuses et ultralumineuses en infrarouge, qui connaissent une activit´e intense, c’est l’´emission de la poussi`ere qui domine la luminosit´e bolom´etrique (alors que la puissance ´emise en optique augmente d’un facteur 4, celle ´emise en infrarouge augmente par 3 ordres de grandeur : Sanders & Mirabel 1996).

— Dans les milieux denses, elle forme une protection efficace contre les photons ultraviolets et optiques et permet l’existence de condensations mol´eculaires tr`es froides, qui donneront ´eventuellement naissance `ades´etoiles. L’´evolution chimique et dynamique des nuages mol´eculaires est coupl´ee `acet´ecrantage du rayonnement (de Boisanger & Chi`eze 1991).

— La poussi`ere joue un rˆole de catalyseur dans la chimie du milieu interstellaire : des atomes et des mol´ecules sont adsorb´es `a la surface des grains, o`u ils peuvent ˆetre pi´eg´es ou mi- grer jusqu’`a rencontrer un autre compos´eadsorb´e avec lequel ils r´eagissent avant d’ˆetre lib´er´es. C’est en particulier le m´ecanisme de formation de l’hydrog`ene mol´eculaire. D’autres mol´ecules se forment par photolyse et recombinaison des constituants du manteau de glaces des grains. II. La poussi`ere dans les galaxies 5

— Les grains de poussi`ere participent sans doute de fa¸con efficace au chauffage du gaz in- terstellaire diffus (Watson 1972). L’ionisation des atomes `a la surface des grains par le rayonnement ultraviolet lointain des ´etoiles s’accompagne d’une lib´eration d’´energie sous la forme d’´energie cin´etique des ´electrons ; de plus, la r´eaction de formation de l’hydrog`ene mol´eculaire sur les grains est fortement exothermique, ce qui permet de lib´erer les mol´ecules nouvellement form´ees avec une grande ´energie cin´etique. Le refroidissement du milieu in- terstellaire diffus s’op`ere par collisions entre ´electrons libres et ions, au cours desquelles une partie de l’´energie cin´etique des ´electrons sert `a exciter un niveau de structure fine de l’ion et est ´evacu´ee par rayonnement. d’Hendecourt & L´eger (1987) et Puget & L´eger (1989) ont propos´e que le type de poussi`ere qui domine l’´emission dans l’infrarouge moyen, sous forme de bandes larges (des macro-mol´ecules de structure planaire dans leur mod`ele), four- nissent un chauffage photo-´electrique plus efficace que les grains de poussi`ere classiques, du fait mˆeme de leur structure. Ils ont ´egalement propos´e un test observationnel, qui est la recherche d’une corr´elation entre la puissance rayonn´ee par la raie de [CII]`a 158 µm, qui est un des plus importants agents de refroidissement du milieu interstellaire atomique, et la puissance rayonn´ee dans les bandes en infrarouge moyen, qui est ´egale `a la puissance ab- sorb´ee par la poussi`ere, principalement dans l’ultraviolet. Ce test a ´et´er´ecemment appliqu´e par Helou et al. (2000) avec succ`es.

II.2 Signatures observationnelles

II.2.1 Absorption et rougissement

L’observation d’une ´etoile de type spectral connua ` de nombreuses longueurs d’onde permet de d´eterminer la loi d’extinction sur sa ligne de vis´ee, en comparant la distribution d’´energie observ´ee `a un spectre synth´etique ou au spectre d’une ´etoile peu rougie de type identique. Un param`etre essentiel, qui d´epend de la nature de la poussi`ere (composition chimique et distri- bution en taille), est le rapport de l’extinction absolue dans une bande sur l’extinction relative entre deux bandes, par exemple RV = A(V )/E(B − V ), o`u A(V ) est l’extinction dans la bande VetE(B − V ) l’exc`es de couleur entre les bandes B et V, c’est `adirelerougissement.Dansle milieu interstellaire diffus, RV est de l’ordre de 3.1, tandis que sur les lignes de vis´ee des ´etoiles enfouies dans un nuage dense ou poss´edant une enveloppe circumstellaire ´epaisse, il atteint en g´en´eral des valeurs plus ´elev´ees. Sch´ematiquement, un param`etre RV plus ´elev´e, c’est `adire une moindre s´electivit´e de l’extinction en fonction de la longueur d’onde, peut ˆetre dˆu`a une distribution en taille favorisant les plus gros grains ; les grains croissent plus facilement par coalescence dans les milieux denses et froids, ce qui explique qu’on observe des rapports RV plus grands en direction des ´etoiles enfouies ou `a enveloppe circumstellaire dense.

La figure II.1 montre trois exemples de courbes d’extinction. Les courbes d’extinction sont tr`es semblables dans la partie proche-infrarouge et optique du spectre, mais peuvent changer drastiquement dans la partie ultraviolette selon la ligne de vis´ee. L’extinction ne varie pas de fa¸con monotone en fonction de la longueur d’onde : il existe des signatures spectrales partic- uli`eres. La plus importante est la large bande autour de 220 nm, qui est attribu´ee `a des compos´es carbon´es de petite taille, sans que leur nature soit ´elucid´ee. Dans le domaine optique, il existe une s´erie de bandes d’absorption fines et peu profondes, appel´ees bandes interstellaires diffuses, 6

calcul observations

Figure II.1: (extraite de Cardelli et al. 1989). Lois d’extinction sur les lignes de vis´ee de trois ´etoiles, repr´esent´ees par des polynˆomes en 1/λ sur diff´erents domaines spectraux, dont l’unique param`etre libre est RV .

Figure II.2: (extraite de Bouchet et al. 1985). Lois d’extinction moyennes de la Galaxie (ligne continue), du Grand Nuage de Magellan (pointill´es) et du Petit Nuage de Magellan (carr´es et triangles). dont l’origine reste inconnue. Enfin, dans l’infrarouge proche et moyen, la glace recouvrant les grains de poussi`ere dans les nuages mol´eculaires produit une absorptiona3.1 ` µm, et les grains de silicates amorphes des bandes larges `a9.7et18µm. Witt et al. (1984) ont montr´equeles variations dans l’intensit´e de la bande d’absorptiona ` 220 nm et dans la pente de la mont´ee en ultraviolet lointain semblent se produire ind´ependamment l’une de l’autre, sugg´erant que deux composantes distinctes de poussi`ere sont responsables de ces signatures.

Les courbes d’extinction moyennes sont ´egalement tr`es diff´erentes d’une galaxie `a l’autre, selon la m´etallicit´e et les conditions physiques des grains. Les lois d’extinction les mieux connues II. La poussi`ere dans les galaxies 7 sont celles de notre Galaxie et des Grand et Petit Nuages de Magellan (Fig. II.2), dont les m´etallicit´es sont en moyenne 1/4et1/10 de la m´etallicit´e solaire. Les diff´erences les plus marqu´ees sont l’intensit´e de la bandea ` 220 nm et la pente de la courbe dans l’ultraviolet lointain. Ces lois d’extinction sont malgr´etoutd´eduites de l’observation d’un tr`es petit nombre d’´etoiles, alors que la loi d’extinction peut varier fortement en fonction de la ligne de vis´ee.

Lorsqu’on observe des r´egions ´etendues non r´esolues en objets individuels, comme un en- semble de r´egions HII et le milieu interstellaire environnant, la g´eom´etrie compliqu´ee des dis- tributions relatives de la poussi`ere et des ´etoiles, ainsi que de possibles gradients spatiaux des propri´et´es physiques de la poussi`ere associ´es aux variations de densit´e de gaz et d’´energie, font que la manipulation de l’extinction est extrˆemement d´elicate. Des rayonnements `a une mˆeme longueur d’onde mais provenant de r´egions diff´erentes, comme par exemple une raie n´ebulaire et le continuum stellaire sous-jacent, ne sont pas affect´es par la mˆeme absorption. De plus, en supposant que l’on consid`ere uniquement des rayonnements de mˆeme origine spatiale, comme diff´erentes raies de recombinaison de l’hydrog`ene, l’extinction affectant un couple de raies `a des petites longueurs d’onde paraˆıtra toujours plus petite que si l’on utilise une plus grande gamme de longueurs d’onde, parce que les r´egions les plus ´eteintes le seront dans les deux raies et contribueront tr`es peu au flux ´emergent dans une raie comme dans l’autre. Pour des r´egions ´etendues, on ne peut donc pas d´eterminer une loi d’extinction, mais plutˆot une loi d’att´enuation effective, qui combine les effets de l’absorption, de la diffusion et les effets g´eom´etriques de distri- bution de la poussi`ere et des ´etoiles. L’absorption peut ˆetre tr`es ´elev´ee sans que la distribution spectrale d’´energie soit significativement rougie (Witt et al. 1992). C’est la raison pour laquelle Calzetti et al. (1994) d´erivent, pour une moyenne de 39 starbursts circumnucl´eaires et galaxies bleues compactes, des “lois d’extinction” tr`es grises.

II.2.2 Rayonnement infrarouge

L’´energie absorb´ee par les grains est principalement r´e´emise dans le domaine infrarouge,a ` des longueurs d’onde comprises entre environ 3 µm et 1 mm. Une “´emission rouge ´etendue”, con- sistant en une tr`es large bande entre environ 6000 et 8000 A,˚ non attribuableaunm´ ` ecanisme de diffusion (Schmidt et al. 1980) est aussi observ´ee dans une grande vari´et´edesites,enpar- ticulier des n´ebuleuses par diffusion, des n´ebuleuses plan´etaires, des r´egion HII et des nuages interstellaires diffus. Des nano-cristaux de silicium sont un bon candidat pour cette ´emission (Ledoux et al. 1998 ; Gordon et al. 2000).

L’´emission en infrarouge lointain, c’est `adirepourλ ≥ 60–100 µm, est produite par des grains de poussi`ere `al’´equilibre thermique dans le champ de rayonnement qui baigne le milieu interstellaire. L’´emissivit´e dans l’infrarouge lointain est usuellement d´ecrite par une loi en λ−n, o`u n vaut entre 1 et 2. L’exposant n est assez mal d´etermin´e, mais Reach et al. (1995) sugg`erent pour rendre compte des observations de la Galaxie par FIRAS, le spectrom`etre en infrarouge lointain (100 µm`a 4.5 mm) qui ´etaita ` bord de COBE, une loi d’´emissivit´e un peu plus complexe, ´equivalentea ` λ−1 pour λ  200 µmetλ−2 pour λ  200 µm. Lorsque le spectre infrarouge des galaxies au-del`a de 100–200 µm est suffisamment bien contraint, il est en g´en´eral incompatible avec une composante de poussi`ere `a une seule temp´erature et implique que de grandes quantit´es de poussi`ere froide existent (T  10 − 20 K) et dominent en masse la composante de poussi`ere 8

1000 100 10 1 0.1 λrest (µm)

Figure II.3: (extraite de Sanders & Mirabel 1996). Distributions spectrales d’´energie moyennes de galaxies s´electionn´ees par leur fluxa60 ` µm. Elles illustrent la pr´edominance de la partie in- frarouge par rapport au reste du spectre lorsque la luminosit´e`a60µm augmente. Cette ´evolution s’accompagne d’un d´eplacement du pic d’´energie vers des longueurs d’onde plus courtes : la temp´erature moyenne de la composante de poussi`ere “ti`ede” augmente.

“ti`ede” (typiquement T  30 − 40 K), seule visible par IRASa ` λ ≤ 100 µm. C’est par exemple la conclusion la plus probablea ` laquelle arrivent Kwan & Xie (1992) en utilisant des donn´ees `a 350 µm et 760 µm en plus des donn´ees d’IRASa ` 60 et 100 µm. Les observations de la Galaxie par FIRAS (Reach et al. 1995) et des galaxies NGC 891 et NGC 3079 avec SCUBA (`a 450 et 850 µm), ces derni`eres combin´ees avec les mesures d’IRAS `a 60 et 100 µm (Alton et al. 1998b ; Stevens & Gear 2000), montrent ´egalement la pr´esence de grandes quantit´es de poussi`ere froide. Trewhella et al. (2000) ont obtenu des observations spectroscopiques entre 50 et 200 µmdecinq galaxies spirales (dont M 51, M 83 et NGC 6946, qui font partie de notre ´echantillon), qui con- firment clairement ce r´esultat.

Le pic d’´energie de l’´emission de la poussi`ere dans les galaxies normales se situe entre 100 et 200 µm. Dans la majorit´e des galaxies s´electionn´ees en optique, la puissance infrarouge totale est de l’ordre d’un tiers de la puissance observ´ee en optique et en ultraviolet, ce qui signifie que la plus grande partie du rayonnement des ´etoiles nous parvient sans ˆetre ´eteint. Le satellite IRAS a cependant r´ev´el´e l’existence d’une classe de galaxies lumineuses et ultralumineuses en infrarouge (voir la revue de Sanders & Mirabel 1996) dans lesquelles la puissance absorb´ee et r´e´emise par la poussi`ere d´epasse celle qui s’´echappe sans ˆetre absorb´ee. Un ensemble de distributions spectrales d’´energie est montr´ee en Fig. II.3. Ces galaxies tr`es lumineuses en infrarouge sont pour la plupart en interaction ou en cours de fusion avec un compagnon, et sont rares dans l’univers local. Leur fr´equence augmente en remontant dans le temps.

Dans l’infrarouge moyen en-dessous de 15 µm, l’´emission des galaxies spirales est g´en´eralement domin´ee par des bandes larges `a 3.3, 6.2, 7.7, 8.6, 11.3 et 12.7 µm, attribu´ees `a des compos´es carbon´es de tr`es petite taille. Ces bandes sont pr´esentes presque partout dans le milieu in- terstellaire,a ` la surface des nuages mol´eculaires, autour des r´egions HII,danslesn´ebuleuses par diffusion, aussi bien dans les nuages diffusa ` haute latitude Galactique (Lemke et al. 1998), et sont aussi d´etect´ees dans les n´ebuleuses plan´etaires (Cohen et al. 1986), les enveloppes de certaines ´etoiles ´evolu´ees et les enveloppes circumstellaires ou disques d’´etoiles pr´e-s´equence II. La poussi`ere dans les galaxies 9 principale. Le fait que leur spectre d’´emission reste quasiment identique dans des champs de rayonnement d’intensit´etr`es variable (mˆeme forme spectrale et rapports d’intensit´e des bandes remarquablement stables) implique que les particulesa ` l’origine de cette ´emission ne sont pas `al’´equilibre thermique avec le champ de rayonnement, mais chauff´ees de fa¸con transitoire par l’absorption d’un seul photon, et qu’elles se d´esexcitent rapidement avant l’arriv´ee d’un nouveau photon.

Le postulat de l’existence de grains de poussi`ere de tr`es petite taille hors-´equilibre et la pr´ediction de leur continuum d’´emission sont ant´erieurs `a leur observation effective (par ex- emple, Aannestad & Kenyon 1979 et les r´ef´erences incluses). Pour un grain de faible capacit´e calorifique, qui passe l’essentiel de son tempsa ` basse temp´erature entre deux absorptions de photons, la temp´erature effective est beaucoup plus ´elev´ee que la temp´erature d’´equilibre corre- spondantaladensit´ ` e de rayonnement ambiante, et le grain rayonnea ` des longueurs d’onde plus petites que s’il ´etaital’´ ` equilibre, d’o`uunexc`es d’´emission en infrarouge moyen. Une ´emission aux alentours de 10 µm correspond `a des fluctuations de temp´erature de quelques centaines de Kelvin.

C’est l’interpr´etation retenue par Sellgren (1984) pour expliquer ses observations du con- tinuum entre 1.25 et 4.8 µmden´ebuleuses par diffusion. Le chauffage peut ˆetre fourni non seulement par le rayonnement stellaire, mais aussi par des r´eactions chimiques exothermiques `a la surface du grain (Allen & Robinson 1975), comme par exemple la combinaison de deux atomes d’hydrog`ene, ou encore par des collisions avec des atomes d’hydrog`ene (ce dernier cas de figure est invoqu´e par Papoular 2000 pour l’excitation des porteurs des bandes d’´emission entre 3et13µm dans la galaxie M 31).

Cohen et al. (1986) ont montr´e que la fraction du flux infrarouge entre 7 et 140 µmprovenant des bandes d’´emission, dans un ensemble de n´ebuleuses plan´etaires, est corr´el´ee avec le rap- port d’abondance C/O. Ce r´esultat ´etaie l’attribution des bandesa ` des compos´es carbon´es. L’identification la plus populaire des porteurs de ces bandes les associea ` des transitions en- tre des niveaux de vibration des liaisons C-H et C-C dans des macro-mol´ecules aromatiques hydrog´en´ees de quelques dizaines d’atomes (L´eger & Puget 1984). Elles sont appel´ees PAH, pour polycyclic aromatic hydrocarbons. Cependant, la constance du spectre des bandes dans des champs de rayonnement ultraviolet variant par plus de trois ordres de grandeur contredit le mod`ele des PAH, qui pr´evoit que les changements de degr´e d’ionisation ont des cons´equences radicales sur les rapports d’intensit´e des bandes (Uchida et al. 2000). De plus, le fait que ces bandes ne soient pas r´esolues en sous-composantes mˆeme `alaplushauter´esolution spectrale ac- cessible, et qu’elles soient tr`es larges avec des ailes ´etendues, favorise des particules de plusieurs centaines plutˆot que quelques dizaines d’atomes (Boulanger et al. 1998).

Des mod`eles alternatifs existent. Papoular et al. (1989) proposent des grains de charbon pr´esentant une structure partiellement cristalline aux plus petites ´echelles, incluant des em- pilements de cycles aromatiques auxquels sont attach´es des groupes fonctionnels qui seraient responsables de l’´emission des bandes infrarouges. Il faut noter que l’id´ee originelle d’associer les bandes infrarougesa ` des groupes fonctionnels attach´es `a des grains carbon´es provient de Duley & Williams (1981). D’autres identifications sont faites avec divers compos´es carbon´es hydrog´en´es et oxyg´en´es, tr`es semblables aux pr´ec´edents mais dont la structure diff`ere dans les d´etails (Borghesi et al. 1987 ; Sakata et al. 1987). Jones & d’Hendecourt (2000) ont r´ecemment 10 propos´e comme responsable des bandes d’´emission infrarouges la structure de surface des nano- diamants, qui comporte des cycles aromatiques, les cœurs de ces mˆemes grains ´etant identifiables aux “tr`es petits grains”, qui ´emettent un continuum thermique au-del`ade10µm(voirci-apr`es).

Dans le mod`ele de Mathis et al. (1977), la loi d’extinction du milieu interstellaire diffus est reproduite par un m´elange ´egal en masse de grains de graphite et de silicates amorphes, leur distribution en taille ´etant approch´ee par une loi de puissance, dN/dr ∝ r−3.5,o`ulerayon ´equivalent r varie de quelques dizainesa ` environ 3000 A.˚ La bande d’absorptiona ` 220 nm est attribu´ee `a des petits grains de graphite (r<150 A).˚ Cependant, ce mod`ele, dans lequel les grains sontal’´ ` equilibre thermique dans le champ de rayonnement interstellaireadestemp´ ` eratures de l’ordre de 20 K, ne rend pas compte de l’´emission infrarouge en-de¸c`ade60µm. Il est n´ecessaire, pour l’expliquer, d’´etendre la distribution en taille jusqu’`a quelques Angstr¨om.

Au-del`ade10µm et en-de¸c`ade60µm, un continuum est en effet visible, largement en exc`es de celui extrapol´e des fluxa60et100 ` µm dans l’hypoth`ese de grains `al’´equilibre thermique. Ce continuum en infrarouge moyen est attribu´e`a des petits grains carbon´es dont le r´egime thermodynamique occupe toute la zone interm´ediaire entre le chauffage purement impulsionnel et l’´equilibre thermique, c’est `a dire que les grains peuvent se refroidir partiellement ou totale- ment dans l’intervalle s´eparant l’arriv´ee de deux photons. Pour rendre comptealafoisdelaloi ` d’extinction moyenne et du spectre d’´emission de la poussi`ere dans le milieu interstellaire dif- fus, D´esert et al. (1990) proposent un mod`ele faisant intervenir trois composantes de poussi`ere : porteurs aromatiques des bandes d’´emission (de rayon inf´erieura10 ` A),˚ tr`es petits grains de structure 3-dimensionnelle et non pas planaire comme les compos´es aromatiques (de rayon com- pris entre environ 10 et 150 A),˚ et gros grainsal’´ ` equilibre thermique (r>150 A).˚ Chacune de ces composantes est responsable dans ce mod`ele d’une partie distincte des spectres d’absorption et d’´emission (Fig. II.4).

Cependant, Uchida et al. (2000) remarquent que le rapport F12/FFIR est remarquablement constant dans les n´ebuleuses par diffusion, pour des ´etoiles excitatrices de temp´eratures tr`es

10-30 / H) 2 cm -21 (W / H atom) ν (10 F ν H σ

10 -33

Figure II.4: (extraite de D´esert et al. 1990). `agauche:Reproduction de la loi d’extinction moyenne (croix) avec trois composantes de poussi`ere (avec RV =3.1 et NH /E(B − V )= 5.81021 H.cm−2). `adroite:Reproduction du spectre d’´emission du milieu interstellaire diffus (croix) par les mˆemes composantes. Les barres horizontales repr´esentent la largeur des filtres photom´etriques. II. La poussi`ere dans les galaxies 11 variables et donc des rayonnements de duret´es tr`es diff´erentes. Cela implique que le mat´eriel responsable de l’´emission `a12µm suit une loi d’extinction coupl´ee `a celle du reste de la poussi`ere qui ´emet dans l’infrarouge lointain. Nous verrons plus loin que l’´emission `a7et15µm est aussi ´etroitement corr´el´ee `al’´emission en infrarouge lointain dans les galaxies.

Unautretypedepoussi`ere produit une ´emission d’avant-plan qui constitue la majeure partie du fond diffus en infrarouge moyen, d’intensit´e parfois comparablea ` celle des galaxies ´etudi´ees ici : la poussi`ere interplan´etaire qui produit la lumi`ere zodiacale par diffusion de photons optiques et r´e´emet dans l’infrarouge l’´energie absorb´ee. Ses sources de production sont principalement les com`etes et ast´ero¨ıdes, mais une petite partie provient ´egalement de poussi`ere interstellaire entrant dans le syst`eme solaire. Le spectre de la poussi`ere zodiacale est facilement reconnaissable et nous verrons plus loin comment le soustraire.

Enfin, les ´etoiles g´eantes froides peuvent aussi contribueral’´ ` emission en infrarouge moyen. Dans l’´echantillon ´etudi´e ici, cette contribution est n´egligeable sauf dans quelques cas isol´es. Elle n’est d´etectable dans les galaxies que lorsque le taux de formation d’´etoiles est tr`es faible. En principe, le spectre d’´emission peut aussi ˆetre absorb´e par des grains de silicates `a9.7et 18 µm. Cependant, cette absorption, pour ˆetre d´etectable, requiert des profondeurs optiques tr`es grandes, qui ne sont rencontr´ees `a grande ´echelle dans aucune des galaxies normales qui constituent notre ´echantillon.

II.3 Formation et destruction des grains de poussi`ere

Les grains de poussi`ere sont compos´es des ´el´ements m´etalliques les plus abondants, en plus de l’hydrog`ene : carbone, azote, oxyg`ene, fer, magn´esium, aluminium, silicium. Ces ´el´ements montrent une d´eficience dans le milieu interstellaire par rapport aux abondances solaires parce qu’une partie importante en est bloqu´ee dans les grains. Les observations d’extinction et des abondances en phase gazeuse impliquent que la majeure partie du silicium et environ les deux tiers du carbone du milieu interstellaire se trouvent dans les grains de poussi`ere.

L’extinction dans le visible est explicable par des grains de silicates de l’ordre de 1000 Ade˚ rayon, constitu´es d’un noyau recouvert d’un manteau de glace contenant de nombreuses im- puret´es (ce manteau existe dans les nuages mol´eculaires seulement). Ce type de grains provoque des bandes d’absorptiona9.7et18 ` µm (silicates) et 3.1 µm (glace).

La poussi`ere est produite dans les vents et les enveloppes froides des ´etoiles ´evolu´ees riches en carbone, en oxyg`ene et en autres ´el´ements synth´etis´es en fin de vie de l’´etoile (g´eantes et superg´eantes rouges, n´ebuleuses plan´etaires, vents des ´etoiles Wolf-Rayet, mais aussi novæ et supernovæ), o`u les ejecta m´etalliques s’associent en mol´ecules qui r´eagissent et s’aggr`egent pour former des grains primaires, hydrocarbones ou silicates. Les silicates sont probablement form´es principalement par les supernovæ et les ´etoiles massives ´evolu´ees riches en oxyg`ene, qui sont les producteurs dominants de silicium (Douvion 2000). Quant au carbone, c’est le premier ´el´ement m´etalliqueaˆ ` etre synth´etis´edansles´etoiles, par combustion de l’h´elium, et on s’attend donca ` une contribution importante des g´eantes de faible masse `a la formation des poussi`eres carbon´ees.

Les grains subissent des transformations sous l’effet du rayonnement local et ceux qui ne sont 12 pas d´etruits sont progressivement dispers´es dans le milieu interstellaire, port´es par le vent de l’´etoile parente. La poussi`ere se sublime dans des champs de rayonnement ultraviolet intense. Les grains peuvent ´egalement ˆetre fragment´es, ´erod´es ou partiellement vaporis´es et leur distribution en taille modifi´ee au passage d’un choc. Ils sont pour une bonne part d´etruits dans les chocs de supernovæ de plusieurs centaines de km.s−1.

Lespetitsgrainssontport´es transitoirementadestemp´ ` eratures de 100–1000 K. L’´energie qu’ils re¸coivent par l’absorption d’un seul photon augmente significativement leur ´energie interne. Par cons´equent, dans les champs de rayonnement intense, seuls les plus gros grains survivent : les porteurs des bandes d’´emission sont d’abord d´etruits avant les autres grains.

L’efficacit´e de destruction des grains, apr`es qu’ils aient ´et´eform´es par les ´etoiles, implique qu’ils se r´eg´en`erent dans le milieu interstellaire, o`u ils croissent par condensation de la phase gazeuse `a leur surface et surtout par coalescence entre grains et accr´etion d’un manteau de glaces (eau, m´ethane, ammoniac, etc.) dans les nuages mol´eculaires. Lorsque les grains sont par la suite expos´es `a un champ de rayonnement plus intense, ils continuenta´ ` evoluer par des r´eactions photochimiques en surface, formant des compos´es organiques plus r´esistants que les glaces (Greenberg 1986).

II.4 Apports d’IRAS `al’´etude des galaxies spirales normales

Le satellite IRAS a effectu´e un relev´e du ciel presque complet dans quatre bandes pho- tom´etriques larges centr´ees `a 12, 25, 60 et 100 µm. Les flux totaux de plus de dix mille galaxies ont ´et´emesur´es dans ces bandes. Je discute bri`evement quelques r´esultats en rapport avec les th`emes abord´es ici.

Les couleurs infrarouges des galaxies se distribuenta ` l’int´erieur d’une bande large o`ules variations du rapport F12/F25 sont inverses de celles du rapport F60/F100 (Helou 1986 ; Fig. II.5). Des galaxies qui forment tr`es peu d’´etoiles occupent la zone du diagrammea ` basse couleur F60/F100 et `arapportF12/F25 ´elev´e. Une augmentation de l’activit´e de formation d’´etoiles a pour effet un d´eplacement dans le diagramme vers la zone sup´erieure gauche. La position d’une galaxie dans ce diagramme est classiquement interpr´et´ee comme r´esultant du m´elange dans des proportions variables de deux composantes de poussi`ere : l’une situ´ee `aproximit´edesr´egions HII et chauff´ee par les ´etoiles massives, l’autre distribu´ee dans le milieu interstellaire neutre diffus et chauff´ee principalement par des ´etoiles de types tardifs (composante appel´ee “cirrus”). Dans le mod`ele propos´e par Helou (1986), les couleurs des galaxies sont reproduites `a l’aide de deux param`etres : la fraction de l’´emission provenant des cirrus (suppos´es de couleurs infrarouges constantes, celles de l’extr´emit´einf´erieure droite du diagramme) et l’intensit´e du rayonnement dans les r´egions actives.

L’utilisation de l’´emission dans l’infrarouge lointain (60–100 µm) en tant qu’indicateur du taux de formation stellaire est controvers´ee. Il est malgr´etoutassezg´en´eralement accept´eque l’infrarouge lointain ne peut remplir le rˆole de traceur quantitatif de la formation stellaire que dans les galaxies formant des ´etoiles `auntauxtr`es soutenu (galaxies starbursts), parce que la composante de poussi`eredutypecirrusestcens´ee repr´esenter la majeure partie de l’´emission infrarouge des galaxies spirales peu actives. Lonsdale-Persson & Helou (1987), par exemple, II. La poussi`ere dans les galaxies 13 60 100 log (F / F )

log (F12 / F25 ) Figure II.5: (extraite de Helou 1986). Diagrammea ` deux couleurs en infrarouge de galaxies ne contenant pas de noyau actif. Les points α et β identifient deux galaxies spirales an´emiques, δ une galaxie starburst, γ une naine bleue compacte et  un syst`eme en interaction. attribuent des fractions de 40a ` 80% de l’´emission infrarouge totale de ce type de galaxiesala ` composante de cirrus ; cependant, le mod`ele utilis´eesttr`es simplifi´e, supposant deux populations de grains `adestemp´eratures fixes. La distinction entre la composante de cirrus et la composante active est arbitraire, en ce sens qu’elle refl`ete surtout des diff´erences dans la distribution spectrale d’´energie du rayonnement de chauffage, et de ce fait d´ependdeladistancedelapoussi`ere aux ´etoiles massives. Loin des sites de formation d’´etoiles, le chauffage est estim´e fourni par des photons optiques provenant des populations stellaires ´evolu´ees. Uchida et al. (1998) ont montr´e la n´ecessit´e d’une contribution importante du rayonnement optique au chauffage des porteurs des bandes aromatiques dans une n´ebuleuse par diffusion illumin´ee par des ´etoiles ´emettant peu de photons ultraviolets. Ce n’est que lorsque l’activit´e de formation d’´etoiles est g´en´eralis´ee `a l’ensemble de la galaxie qu’il est certain que le chauffage des grains soit essentiellement dˆuau rayonnement ultraviolet des ´etoiles jeunes.

La raie de recombinaison Hα est usuellement utilis´ee comme traceur des ´etoiles plus massives que 10 M dans les galaxies, moyennant certaines hypoth`eses sur les conditions de densit´eet de temp´erature dans les r´egions HII (Kennicutt 1998a). Sauvage & Thuan (1992) ont montr´e que dans un ´echantillon de galaxies spirales s´electionn´ees optiquement, moins actives que des galaxies s´electionn´ees en infrarouge, la relation entre les luminosit´es en infrarouge lointain et en 1.45 Hα est non-lin´eaire (LFIR ∝ LHα ). De plus, l’exc`es infrarouge, c’est `a dire l’exc`es du rapport LFIR/LHα relativementa ` la valeur attendue si l’´emission infrarouge ´etait uniquement aliment´ee par les ´etoilesjeunes,varielelongdelas´equence spirale de Hubble, comme indiqu´eparlafig- ure II.6. Cette progression s’interpr`ete le plus naturellement comme une contribution croissante de la poussi`eredetypecirrus`al’´emission infrarouge, en moyenne, des types morphologiques tardifs aux types pr´ecoces. Nous y reviendrons ult´erieurement.

La grande base de donn´ees produite par IRAS a ´egalement serviades´ ` etudes statistiques de l’influence de la morphologie barr´ee sur l’´emission infrarouge des galaxies. Comme discut´edans le chapitre suivant, l’existence d’une barre peut ˆetre un moyen efficace d’amener de grandes quan- 14

Figure II.6: (extraite de Sauvage & Thuan 1992). Rapport moyen des luminosit´es en in- frarouge lointain et dans les raies voisines Hα et [NII], en fonction du type morphologique, d’un ´echantillon de 135 galaxies. Le rapport de la raie Hα aux raies [NII], qui ne sont pas s´epar´ees dans des observationsa ` basse r´esolution spectroscopique, est assez stable dans les dis- ques de galaxies (Kennicutt & Kent 1983). Les barres d’erreur repr´esentent la dispersion `a1σ `a l’int´erieur de chaque type. La ligne continue indique la valeur mesur´ee dans les sites de for- mations d’´etoiles de la galaxie Scd M 33 o`u la poussi`ere est la plus chaude, et la ligne pointill´ee la valeur r´esultant de la combinaison de deux calibrations pour les taux de formation d’´etoiles, l’une en termes de luminosit´eHα et la deuxi`eme en termes de luminosit´e infrarouge.

tit´es de gaz au centre d’une galaxie, ce qui favorise le d´eclenchement d’une formation d’´etoiles intense. Hawarden et al. (1986) ont inspect´e les couleurs infrarouges d’un large ´echantillon de galaxies spirales s´electionn´ees optiquement et d´etect´ees dans les quatre bandes d’IRAS, ne con- tenant pas de noyau actif. Ils trouvent qu’une fraction importante des galaxies fortement barr´ees, ainsi que quelques galaxies faiblement barr´ees, ont un exc`es d’´emission dans la bandea25 ` µm, relativemental’´ ` emission `a 12 et 100 µm, par rapport aux couleurs observ´ees dans les galaxies non barr´ees. Comme des rapports F25/F12 ´elev´es sont caract´eristiques de poussi`ere chaude dans des r´egions de formation d’´etoiles, l’exc`es `a25µm des galaxies barr´ees est attribuable `a une contribution accrue des sites de formation d’´etoiles `al’´emission infrarouge totale. L’´echantillon utilis´e par Hawarden et al. (1986) devant satisfaire la contrainte d’une d´etection dans toutes les bandes d’IRAS, il est biais´e vers des rapports de luminosit´es infrarouge sur optique plus grands qu’un ´echantillon s´electionn´e purement sur des crit`eres optiques. Huang et al. (1996) ont reconsid´er´elaquestionencomparanttrois´echantillons s´electionn´es de mani`eres diff´erentes, l’un compos´e de galaxies brillantes `a60µm, le deuxi`eme similaire `al’´echantillon utilis´epar Hawarden et al. (1986), et le dernier limit´e en distance uniquement (´echantillon provenant de Isobe & Feigelson 1992). Ils arrivent `a la conclusion que seules les galaxies fortement barr´ees de types pr´ecoces (entre S0/a et Sbc), et de luminosit´e infrarouge relativea ` la luminosit´ebleue (LFIR/LB)sup´erieure `a un seuil d’environ 0.3, sont sujettesa ` des exc`es d’´emission `a25µm. Con- trairementa ` Hawarden et al. (1986), Huang et al. (1996) n’observent pas de diff´erence entre les cat´egories de galaxies non barr´ees et faiblement barr´ees. II. La poussi`ere dans les galaxies 15

II.5 Centres galactiques observ´es en infrarouge

Les t´elescopes au sol permettent d’atteindre une bien meilleure r´esolution angulaire en in- frarouge et en particulier d’observer les r´egions centrales des galaxies. Devereux (1987) a com- bin´edesmesures`a10µm dans une petite ouverture plac´ee sur des noyaux galactiques avec les mesures `a12µmd’IRASetaainsid´emontr´e que la couleur F25/F12 est corr´el´ee avec le degr´e de compacit´edel’´emission `a10µm. Il confirme de plus que les galaxies les plus actives (c’est `adireavecunexc`es de couleur et de compacit´e) sont barr´ees et de type pr´ecoce, et leur noyau est classifi´e comme noyau HII ou noyau de Seyfert.

Telesco et al. (1993) ont cartographi´e les centres de galaxies starbursts brillantes en in- frarouge dans un filtre largea ` 10.8 µm, avec une r´esolution angulaire de l’ordre de 4.Les morphologies sont diverses, montrant des pics intenses dans le noyau en absence de r´esonance interne de Lindblad, des anneaux ou des concentrations extranucl´eaires asym´etriques si des r´esonances existent. Par ailleurs, ces sources centrales brillantes en infrarouge sont li´ees `ades concentrations de gaz mol´eculaire, qui sont toutefois en g´en´eral plus ´etendues. Un autre r´esultat important de Telesco et al. (1993) est que le chauffage collisionnel de la poussi`ere dans les chocs de supernovæ, par des collisions avec le gaz, est n´egligeable devant le chauffage radiatif. Le fait que la poussi`ere ´emettant `a 10.8 µm soit chauff´ee par des ´etoiles massives dans les starbursts circumnucl´eaires est attest´e par la bonne corr´elation existant entre la luminosit´e`a 10.8 µmetla luminosit´e dans la raie de recombinaison Brγ `a2.17µm, qui subit une extinction mod´er´ee.

Cependant, ils attribuent l’existence de gradients de la couleur F19.2/F10.8 dans certains cen- tres de galaxies `a sursaut de formation d’´etoiles, d´ecroissante du centre vers la p´eriph´erie, `a l’effet suivant : une augmentation de F19.2/F10.8 ´etant due selon eux `a une baisse de temp´erature de la poussi`ere (c’est le comportement d’un corps noir), ils l’interpr`etent comme l’indice de la de- struction des tr`es petits grains ´emettant en infrarouge moyen, par le champ de rayonnement ultraviolet intense au cœur du starburst. L’information spectroscopique obtenue avec les in- struments `a bord d’ISO conduita ` une interpr´etation oppos´ee : lorsque l’intensit´educhamp de rayonnement augmente, le continuum des petits grains se d´eplace vers les courtes longueurs d’onde, mais n’est visible qu’au-del`ade10µm dans le filtre large centr´e`a 10.8 µm (parce que les grains ne peuvent pas atteindre des temp´eratures sup´erieures `a leur temp´erature de subli- mation), et augmente plus vite `a 19.2 qu’`a 10.8 µm. De 6 `a13µm, on observe toujours dans les starbursts une deuxi`eme sorte de poussi`ere, les bandes d’´emission aromatiques (par exemple Genzel et al. 1998 ; Laurent et al. 2000). Des couleurs F19.2/F10.8 plus ´elev´ees dans les centres que dans les zones circumnucl´eaires indiquent donc bien un champ de rayonnement plus intense, mais sans disparition des petits grains, et une “temp´erature” moyenne de ces grains plus ´elev´ee. 16 17

Chapitre III Les galaxies spirales barr´ees

III.1 Un bref historique

Les galaxies barr´ees ont ´et´edistingu´ees des galaxies spirales ordinaires pour la premi`ere fois en 1918 par H.D. Curtis, avant mˆeme que les galaxies ne soient identifi´ees `adessyst`emes stellaires similaires et ext´erieurs `alaVoielact´ee.

Les “n´ebuleuses blanches” (c’est la d´esignation des galaxies avant qu’elles ne soient recon- nues comme telles) ont ´et´eobserv´ees d`es le dix-huiti`eme si`ecle. A cette ´epoque, seules des sp´eculations philosophiques sur leur nature ´etaient possibles, telle l’hypoth`ese des univers-ˆıles. Il est remarquable que cette th´eorie, bien avant que des preuves observationnelles puissent ˆetre apport´ees, ait postul´e la finitude de notre Galaxie et l’existence de syst`emes stellaires semblables isol´es les uns des autres. Des catalogues de n´ebuleuses et amas stellaires, m´elangeant les sources Galactiques et les galaxies externes, ont vu le jour aux dix-huiti`eme et dix-neuvi`eme si`ecles. Le catalogue NGC (New General Catalogue of nebulæ and clusters of stars) et ses suppl´ements IC (Index Catalogues), compil´es par J.L.E. Dreyer en 1888-1908, sont toujours intensivement utilis´es aujourd’hui. La structure spirale dans les n´ebuleuses a ´et´ed´ecouverte d`es 1845 par Lord Rosse.

Le d´ebut du si`ecle, jusqu’en 1924, a ´et´emarqu´e par une vive controverse sur la nature des n´ebuleuses blanches, due `a l’ind´etermination de leurs distances : faisaient-elles partie de la Voie lact´ee, auquel cas elles n’´etaient pas constitu´ees d’´etoiles mais ´etaient de vraies n´ebulosit´es, ou bien ´etaient-elles des objets ext´erieurs, rejoignant l’id´ee d’univers-ˆıles ? La d´ecouverte de novæ dans plusieurs n´ebuleuses ne permit pas de trancher le d´ebat, car s’y mˆelaient des su- pernovæ beaucoup plus lumineuses, maintenant reconnues comme une classe d’objets distincte. L’identification des sources ponctuelles observ´ees dans les galaxies les plus proches `ades´etoiles, renforc´ee par des observations spectroscopiques, n’´etaient pas toujours prises au s´erieux parce que ces sources pr´esentaient un aspect plus n´ebulaire que les ´etoiles connues. Le probl`eme fut r´esolu en 1924 par E. Hubble, par la d´ecouverte dans plusieurs n´ebuleuses d’´etoiles de la classe des c´eph´eides, dont la relation p´eriode de variabilit´e-luminosit´e, ´etablie par Henrietta Leavitt pour les c´eph´eides des Nuages de Magellan, permet d’estimer les distances.

Les galaxies pr´esentent une tr`es grande vari´et´e de structures et chacune peut ˆetre consid´er´ee comme unique. Cependant, le besoin de d´efinir des ´el´ements morphologiques caract´eristiques et de simplifier cette complexit´es’esttr`es vite fait sentir. La classification propos´ee par E. Hubble, 18 notamment, dans laquelle les galaxies sont ordonn´ees selon une s´equence allant de types dits pr´ecoces vers des types dits tardifs, groupe commod´ement les galaxies dans le but de lier leurs formes `adespropri´et´es physiques et `a une possible s´equence d’´evolution. Il est d’ailleursanoter ` que si ´evolution il y a le long de la s´equence morphologique, les id´ees actuelles indiquent qu’elle suit plus certainement une s´equence invers´ee, des types tardifs vers les types pr´ecoces. Plusieurs syst`emes de classification existent, mais le plus utilis´e est certainement celui de E. Hubble, r´evis´e et ´etendu par lui-mˆeme, G. de Vaucouleurs, et d’autres. Du fait de son caract`ere de simplicit´e, il permet de n´egliger des d´etails qui n’ont pas forc´ement une grande signification physique ou dynamique. Cependant, il ne d´ecrit pas les galaxies “particuli`eres” (galaxies en interaction, galaxies amorphes, galaxies cD au centre des amas, galaxies naines, etc.), et ne rend pas compte de la nature et de la force des ondes spirales (bras r´eguliers, multiples ou filamentaires, massifs ou fins).

Van den Bergh (1976) a distingu´e une nouvelle classe de galaxies spirales appel´ees an´emiques, d´efinie comme une s´equence parall`ele `a celle des spirales normales, interm´ediaire entre celles-ci et les galaxies lenticulaires (Fig. III.1). Dans le syst`eme de van den Bergh (1976), les lenticulaires ne sont pas en effet des objets de transition entre les elliptiques et les spirales, tels que les ad´efinis E. Hubble, mais des analogues des spirales, class´ees de S0a `a S0c, en accord avec la constatation que les galaxies class´ees lenticulaires ont des rapports bulbe sur disque tr`es variables. Les diff´erences entre les trois s´equences sont attribu´ees `a des variations de richesse en gaz et de formation d’´etoiles. Van den Bergh (1976) fait le lien entre la fr´equence des galaxies lenticulaires et an´emiques avec le type d’environnement (champ ou amas). Le meilleur moyen de quantifier l’an´emie d’une galaxie, reconnue comme tellea ` partir de photographies en bande bleue, est d’ailleurs certainement de mesurer la d´eficience en gaz HI, qui affecte des galaxies en interaction avec le gaz intra-amas.

Dans le syst`eme de Hubble, le type d’une galaxie spirale (de S0/aa ` Sdm) est d´efini par trois crit`eres, subjectifs en l’absence de quantification : – le rapport de taille du bulbe et du disque (croissant des types tardifs vers les types pr´ecoces). – l’enroulement des bras spiraux sur eux-mˆemes (idem). –ledegr´eder´esolution des bras en ´etoiles et r´egions HII (croissant des types pr´ecoces vers les

Figure III.1: (extraite de van den Bergh 1976). Sch´emadelaclassificationpropos´ee par van den Bergh (1976). S0 est le symbole des galaxies lenticulaires, A des galaxies an´emiques et S des spirales classiques. III. Les galaxies spirales barr´ees 19

types tardifs). Dans la pratique, le troisi`eme crit`ere prime fortement sur les deux autres (nous y reviendrons), et Sandage (1961) note que les galaxies Sa ont un bulbe de taille tr`es variable, souvent aussi petit que dans les galaxies plus tardives, le premier crit`ere de classification n’´etant pas n´ecessairement en accord avec les deux suivants. Freeman (1970) a montr´e que le rapport de la luminosit´ebleue du bulbe sur la luminosit´e totale et le rapport de taille du bulbe et du disque ne sont que tr`es faiblement corr´el´es avec le type morphologique de de Vaucouleurs. Seigar & James (1998), par exemple, ont confirm´e qu’il n’existe pas de relation univoque entre le rapport massique bulbe sur disque et le type morphologique, en utilisant des observations en infrarouge proche de mani`ere `a quantifier la masse stellaire.

Malgr´e la subjectivit´e de la classification, la s´equence de Hubble rend compte dans les grandes lignes d’une s´equence de propri´et´es physiques (notamment quantit´e de gaz HI et type dominant de populations stellaires). Il est vrai aussi que de plus en plus d’´etudes r´ecentes tendent `a montrer qu’il peut exister une formation stellaire importante au centre des galaxies pr´ecoces (Hameed & Devereux 1999). La s´equence morphologique ne semble pas ˆetre interpr´etable en termes de formation d’´etoiles globale (`alafoisdansledisqueetlesr´egions centrales), mais plutˆot en termes de formation d’´etoiles dans le disque uniquement.

Les barres ont pris une place de plus en plus importante dans la classification au cours des ann´ees. On a d’abord r´ealis´e qu’il existe une continuit´e entre galaxies ordinaires et barr´ees : les barres sont de taille et de forme tr`es variables. C’est ce qui a incit´e G. de Vaucouleursa ` introduire des s´equences interm´ediaires entre celle des spirales ordinaires et celle des spirales fortement barr´ees. Un grand nombre de galaxies mod´er´ement barr´ees sont class´ees comme ordinaires dans Sandage (1961) et Sandage & Bedke (1994), mais reconnues comme barr´ees dans le RC3 (de Vaucouleurs et al. 1991). Par ailleurs, la morphologie d’une galaxie d´epend fortement de la longueur d’ondea ` laquelle elle est observ´ee, `a cause de variations dans le type des populations stellaires et dans l’extinction. En effet, les ´etoiles ´evolu´ees, qui constituent l’essentiel de la masse stellaire, forment une distribution dense et lisse, qui d´efinit le potentiel gravitationnel et en particulier celui d’une barre ; les ´etoiles jeunes ont une distribution plus fragmentaire, en amas, concentr´ee dans les zones de compression du gaz par les ondes de densit´e, et leur morphologie est plus d´eform´ee par l’absorption duealapoussi` ` ere. Ainsi, la fr´equence de d´etection des barres augmente du bleu au rouge eta ` l’infrarouge proche.

La morphologie barr´ee n’est donc pas une d´eviation `ala“normalit´e” que constitueraient les galaxies d´epourvues de barre. En effet, elle est observ´ee dans une proportion qui varie entre un tiers et plus de deux tiers des galaxies spirales selon la force de la barre. En s´electionnant dans le catalogue RC3 (Third Reference Catalogue of Bright Galaxies, de Vaucouleurs et al. 1991) les spirales dont l’ellipticit´e des isophotes externes ne d´epasse pas 0.5 (ce qui garantit que leur inclinaison sur la ligne de vis´ee est faible) et dont la magnitude bleue totale est plus petite que 14, afin de r´eduire la fr´equence des classifications morphologiques incertaines, on obtient cette distribution : 36% sont fortement barr´ees et 32% montrent une distorsion barr´ee moins prononc´ee (classes respectives SB et SAB). Ces chiffres repr´esentent en fait des limites inf´erieures, car les types morphologiques impr´ecis ont ´et´eaffect´es `a la classe des spirales non barr´ees (SA), et des observables mieux choisies que la lumi`ere bleue (par exemple le rayonnement en infrarouge proche, qui est un meilleur indicateur de la masse stellaire totale, ou bien l’´emission du gaz 20 mol´eculaire) permettraient de d´etecter davantage de barres, y compris les barres nucl´eaires.

En parall`ele, les ´etudes de la dynamique des galaxies ont montr´e qu’une structure barr´ee a tendancea ` se former spontan´ement dans la plupart des disques, ce qui implique une grande fr´equence de barres effectivement observ´ee. Un disque est stabilis´e contre la formation d’une barre par un halo massifa ` l’int´erieur des limites du disque (un halo rend le disque plus faiblement auto-gravitant, et donc plus stable : Ostriker & Peebles 1973), et une faible perturbation est empˆech´ee de croˆıtre par un bulbe compact (qui fait que la courbe de rotation monte rapidement au centre, et qu’une r´esonance interne de Lindblad se d´eveloppe : Sellwood 1989). Dans les ann´ees 60, on trouve l’hypoth`ese que le gaz est ´eject´elelongdelabarre,ducentreversl’ext´erieur, alimentant les bras spiraux (de Vaucouleurs 1962 ; Freeman 1965), sur la base d’observations cin´ematiques incompl`etes par spectroscopie `a fente. Huntley (1978) a mod´elis´elemouvement du gaz dans la galaxie barr´ee NGC 5383 par des simulations hydrodynamiques et obtenu des trajectoires elliptiques, qui procurent un meilleur ajustement aux observations cin´ematiques que d’autres mod`eles non coplanaires. Roberts et al. (1979),a ` partir de nouvelles simulations hydrodynamiques, ont montr´equ’enr´eponse `a une barre, des chocs se d´eveloppent du cˆot´e aval de la barre et,a ` cause de sa nature dissipative, le gaz d´erive lentement vers le centre `a chaque r´evolution ; ils insistent sur le fait que mˆeme une barre de faible amplitude est capable de perturber fortement la composante gazeuse.

III.2 Aspects morphologiques

Une barre est une structure allong´ee au centre des galaxies, plus ou moins en forme de cigare ou d’ovale, aux extr´emit´es de laquelle se connectent les bras spiraux. Elle est en g´en´eral plus ais´ement observable dans le rayonnement des ´etoiles vieilles qu’en lumi`ere bleue, mais certaines barres nucl´eaires sont mises en ´evidence par la distribution et la cin´ematique du gaz. Trois exemples de galaxies spirales barr´ees, diff´erant dans la mani`ere dont les bras se d´eveloppent `a partir de la barre, sont montr´es dans la figure III.2. Le type (s) d´esigne des bras spiraux partant des extr´emit´es de la barre, le type (r) un anneau interne, les bras partant tangentiellementa ` l’anneau, et le type (rs) une structure de transition.

Prendergast (1983) note que plus la barre est forte, plus l’angle entre la barre et les bras est marqu´e : les deux structures sont dans la continuit´e l’une de l’autre dans le cas d’une barre faible, et quasiment perpendiculaires dans le cas d’une barre forte. La notion de force de la barre est dynamique, mais elle est le plus souvent estim´ee `a partir de la seule morphologie pour des raisons de simplicit´e. La force d’une barre est notamment suppos´ee reli´ee au rapport de taille de son axe majeur et de son axe mineur, oua ` son extension par rapporta ` la taille du disque (voir par exemple Martin 1995), solutions qui ne sont pas enti`erement satisfaisantes. L’efficacit´e d’une barre pour provoquer des transferts radiaux de masse est plutˆot d´etermin´ee par le contraste entre la perturbation du potentiel gravitationnel, et la partie du potentielasym´ ` etrie axiale. L’estimation de ces quantit´es d´epend de l’estimation des masses stellaires. Buta & Block (2001) ont d´efini une mani`ere satisfaisante du point de vue dynamique de quantifier la force de la barre `a partir d’images en infrarouge proche, et trouvent que leur param`etre est corr´el´e avec les classes A, AB et B de G. de Vaucouleurs, cependant avec un chevauchement non n´egligeable de ces trois classes. III. Les galaxies spirales barr´ees 21

Figure III.2: (source : A.R. Sandage et G.A. Tammann, A revised Shapley-Ames Catalogue, Carnegie Institution of Washington). Trois galaxies de mˆeme type de Hubble avec des structures barre-bras diff´erentes. De haut en bas : NGC 4304 (SB(s)bc), NGC 613 (SB(rs)bc) et NGC 2523 (SB(r)bc). 22

Le contenu en gaz et en ´etoiles jeunes des barres d´epend fortement du type morphologique. Les barres des galaxies spirales tardives sont souvent bien d´elimit´ees par l’´emission dans la raie Hα, tandis que cette ´emission se r´eduit le plus souvent dans les barres des spirales pr´ecoces `a de rares complexes brillants entour´es par une structure diffuse (Garc´ıa-Barreto et al. 1996). Sandage (1961) note aussi que la barre est r´esolue en r´egions brillantes dans les galaxies de type SBc, contrairementa ` celles de types SBa et SBb. Des exemples de cartes Hα montrant cela pour quelques galaxies de l’´echantillon que j’ai ´etudi´e sont incluses dans le chapitre V, et compar´ees aux cartes en infrarouge moyen.

D’apr`es leurs simulations num´eriques incluant la formation d’´etoiles (le mod`ele, qui est d´ecrit dans Friedli & Benz 1995, part d’une configuration initiale axisym´etrique, le gaz ´etant distribu´e dans un disque homog`ene), Martin & Friedli (1997) remarquent que les barres rem- plies de r´egions HII semblent d´epourvues de r´esonances internes de Lindblad (c’est `a dire qu’elles n’abritent pas de concentration de masse importante dans les r´egions nucl´eaires), et sont prob- ablement plus jeunes que les barres dans lesquelles la formation d’´etoiles ne se produit qu’au centre. L’´evolution g´en´erale de la formation d’´etoiles dans les barres serait donc la suivante : distribution initiale tout le long de la barre (similairea ` celle de la formation d’´etoiles dans les bras spiraux) ; puis d´ebut d’une concentration circumnucl´eaire ; puis ´epuisement de la formation d’´etoiles le long de la barre et renforcement de la concentration circumnucl´eaire.

La longueur des barres, normalis´ee par la taille du disque, semble conditionner la distinction entre les classes SAB et SB (il y a du moins un bon accord entre ces deux propri´et´es pour les galaxies de notre ´echantillon). Les bras ont aussi tendancea ` se connecter aux barres longues `a angle droit, et aux barres courtes de fa¸con continue. La longueur des barres est de plus anti-corr´el´ee `a leur rapport d’axes (Martinet & Friedli 1997), et c’est en premi`ere approxima- tion une mesure de leur aptitudea ` collecter le gaz du disque. Elle tend `aaugmenteravecla taille des bulbes (Athanassoula & Martinet 1980 ; Martin 1995). Cependant, comme rappel´e pr´ec´edemment, le rapport de taille ou de masse du bulbe et du disque n’est que faiblement reli´e au type morphologique. Par ailleurs, Seigar & James (1998) ont d´efini un param`etre des barres (mesur´e sur des images en infrarouge proche) qu’ils appellent l’angle ´equivalent, par analogie avec la largeur ´equivalente des raies spectrales, et qui est un estimateur de la force des barres plus pr´ecis que leur longueur normalis´ee. Ils trouvent que des barres faibles se rencontrent dans tout l’intervalle de rapport bulbe sur disque de leur ´echantillon de galaxies, mais que les barres fortes tendent `aˆetre li´ees `a des rapports bulbe sur disque relativement ´elev´es.

Comme dans les bras spiraux, des bandes de poussi`ere sont souvent visibles le long des barres, et r´ev`elent des zones de grande densit´e du gaz interstellaire. Elles ont des formes caract´eristiques, qui d´ecoulent de la dynamique du gaz, et qui sont bien reproduites par les simulations hydrody- namiques d’Athanassoula (1992b) (voir ci-dessous). Des arcs de poussi`ere perpendiculaires aux bandes principales peuvent aussi exister (particuli`erement bien visibles dans NGC 1097).

Les barres sont des ondes de densit´edesym´etrie d’angle π, qui tournent en bloc avec une vitesse angulaire caract´eristique, et s’identifienta ` des ondes spirales de grande ampli- tude. Elles g´en`erent en effet une structure spirale bien d´evelopp´ee et r´eguli`ere (avec deux bras dominants), qui peut aussi ˆetre induite par des interactions de mar´ee. Les simulations de Combes & Gerin (1985), par exemple, o`u les nuages mol´eculaires sont trait´es comme des par- ticules `a collisions in´elastiques, montrent la r´eponse spirale du gaza ` une barre. A l’inverse, une III. Les galaxies spirales barr´ees 23 substantielle fraction des galaxies ordinaires isol´ees ont une structure spirale filamentaire, faite de fragments de bras multiples d’aspect chaotique, cr´e´es par une instabilit´evis`a vis d’ondes de sym´etries d’ordres plus ´elev´es.

III.3 Dynamique du gaz

La r´eponse du gaza ` une barre d´epend beaucoup des r´esonances de l’onde barr´ee avec le potentiel non perturb´e. Leur existence et leur emplacement peuvent ˆetre d´eduits d’observations cin´ematiques du gaz. La moyenne azimuthale de la courbe de rotation observ´ee donne une estimation de la courbe de rotation qui serait observ´ee en l’absence de perturbation, d´etermin´ee par la composante axisym´etrique du potentiel. La phase de l’onde barr´ee peut s’´ecrire ωt−mθ = m [Ωb −Ω(r)]t o`u le nombre d’onde m est ´egal `a2,Ω(r)d´esigne la courbe de rotation angulaire axisym´etrique et Ωb la vitesse angulaire de l’onde. Dans un r´ef´erentiel o`ulabarreestaurepos, les ´etoiles tournent `alavitesseΩ(r) − Ωb,retrouvantlamˆeme configurationa ` une fr´equence m r − / π [Ω( ) Ωb ] (2 ), et d´ecrivent de petites oscillations autour de
la composante circulaire de leur trajectoire, les ´epicycles,a ` la vitesse angulaire κ(r)=2Ω(r) 1+0.5 dlnΩ/d ln r .Les r´esonances sont d´efinies par

m [Ω(r) − Ωb ]=lκ(r) (III.1) o`u l est un entier (Athanassoula 1984). Ω(r)=Ωb corresponda ` la corotation (l =0)etl = ±1 aux r´esonances de Lindblad (interne pour le signe + et externe pour le signe −). La figure III.3 est une illustration sch´ematique de deux cas de figure avec ou sans r´esonance interne de Lindblad. Lorsque la concentration de masse au centre de la galaxie augmente, la courbe de rotation croˆıt plus rapidement, ce qui facilite la pr´esence d’une r´esonance interne de Lindblad.

L’´elongation des structures stellaire et gazeuse, due `a la forme quasi-elliptique de la princi- pale famille d’orbites p´eriodiques, est la signature d’une perturbation de grande amplitude du potentiel gravitationnel axisym´etrique du disque. La vitesse de rotation de l’onde barr´ee est plus petite que celle du gaz, et les deux vitesses sont ´egales `aunrayonder´esonance (corotation) situ´e au-del`a des extr´emit´es de la barre. Cette perturbation peut apparaˆıtre spontan´ement dans un disque dynamiquement “ti`ede”, c’est `a dire dont la dispersion de vitesse σ des ´etoiles est Q σκ µ faible ; cela revienta ` dire que le param`etre de stabilit´edeToomre, = πGµ,o`u est la densit´e surfacique de masse, est un peu sup´erieur `a 1, garantissant la stabilit´e contre l’effondrement radial, mais autorisant la croissance de perturbations. Elle peut ´egalement ˆetre induite par in- teraction de mar´ee avec une galaxie compagnon, car on a remarqu´e une plus grande proportion de galaxies barr´ees dans les syst`emes binaires que dans le champ ou les amas, du moins pour les types pr´ecoces (Elmegreen et al. 1990).

Adesr´esonances entre l’onde de densit´e et le potentiel non perturb´epeuventˆetre associ´ees des signatures morphologiques : des anneaux (ou pseudo-anneaux s’ils sont constitu´es par des bras spiraux tr`es enroul´es) de forme elliptique, qui sont suppos´es ˆetre situ´es `aproximit´edela r´esonance interne de Lindblad 2/1 pour les anneaux nucl´eaires, de la corotation (plus pr´ecis´ement de la r´esonance ultraharmonique 4/1) pour les anneaux internes, ou de la r´esonance externe de Lindblad pour les anneaux externes, dans l’ordre croissant de la distance au centre. Les anneaux 24 nucl´eaires sont souvent tr`es inhomog`enes et principalement form´es d’une collection de r´egions HII g´eantes. Ils sont d’ailleurs plus facilement observables dans les traceurs du gaz que dans le rayonnement stellaire.

Au franchissement de chaque r´esonance, l’orientation des orbites p´eriodiques dominantes et de l’´eventuel anneau correspondant change : elle est alternativement parall`ele ou perpendiculaire `a la barre (parall`ele entre la r´esonance interne de Lindblad si elle existe, ou le centre sinon, et la corotation). La formation d’anneaux dans les galaxies barr´ees est bien comprise : ils r´esultent du pi´egeage et de l’accumulation de gaz qui a migr´e dans le disque par perte de moment angulaire, dˆu au couple gravitationnel exerc´e par la barre. Les anneaux circumnucl´eaires semblent se trouver exclusivement dans les types pr´ecoces (avant SBbc), c’est `a dire dans des galaxies ayant vraisemblablement une densit´e centrale de masse suffisante pour induire des r´esonances internes de Lindblad.

Des anneaux peuvent aussi ˆetre pr´esents dans des galaxies apparemment non barr´ees (`amoins qu’elles n’aient une faible distorsion ovale non d´etect´ee), mais il pourrait s’agir dans ce cas d’une survivance `a une barre d´etruite (Athanassoula 1996).

Les lentilles, fr´equemment observ´ees dans les spirales barr´ees pr´ecoces, qui sont des struc- tures ovales `abordtranch´eetdontladensit´e de luminosit´e interne est relativement uniforme (dont quelques exemplaires observ´es en infrarouge moyen sont visibles dans l’Atlas, par exem- ple NGC 1672 et NGC 1097), pourraient r´esulter de la dissipation partielle de la barre par une concentration de masse centrale (Combes 1996). Leur origine est cependant controvers´ee.

La barre est soutenue par une famille principale d’orbites stellaires p´eriodiques appel´ee x1, dont l’´elongation est parall`ele `a l’axe majeur de la barre. Lorsque des r´esonances internes de Lindblad existent, des orbites p´eriodiques x2,d’´elongation parall`ele `a l’axe mineur de la barre,

Figure III.3: Exemple de courbe de rotation et de r´esonances. `agauche:Vitesse lin´eaire du gaz(traitcontinu)etdedeuxr´ealisations diff´erentes d’une barre (tirets). `adroite:Vitesse angulaire moyenne Ω,fonctionsΩ−κ/2 et Ω+κ/2 et leurs intersections avec la vitesse angulaire de la barre Ωb. Dans le cas Ωb1, il n’y a pas de r´esonance interne de Lindblad ; dans le cas Ωb2, il en existe deux. III. Les galaxies spirales barr´ees 25

Figure III.4: (extraite de Regan et al. 1999). `agauche:Sch´ema des orbites du gaz lorsqu’il est trait´e non comme un fluide mais comme un ensemble de nuages dissipant de l’´energie unique- ment dans les collisions entre eux. `adroite:Champ de vitesse du gaz dans le r´ef´erentiel de la barre d’un mod`ele hydrodynamique. La densit´edegazestrepr´esent´ee en niveaux de gris. Les fines bandes horizontales, qui correspondent aux bandes de poussi`ere, sont l’emplacement de chocs. se d´eveloppent au centre (Athanassoula 1992a). Le gaz ne suit pas ces orbites p´eriodiquesa ` cause de sa nature dissipative. Du fait de son d´ephasage avec le potentiel (sa vitesse de rotation est plus grande que celle de la barre), il subit un couple gravitationnel et circule sur des orbites elliptiques passant graduellement de la forme des orbites x1 `a celle des orbites x2,delapartie externe vers la partie interne de la barre. Du cˆot´e de la barre en aval de l’axe majeur (c’esta ` dire le cˆot´e qui voit arriver le gaz venant de l’apocentre des orbites x1 et qui le voit partir vers le p´ericentre, “leading side” en anglais), le gaz circulant sur diff´erentes trajectoires se rejoint (il y a surpeuplement d’orbites, “orbit crowding”), et des chocs se d´eveloppent `a grande ´echelle. La figure III.4 montre une repr´esentation du champ de vitesse du gaz et des bandes de choc dans un mod`ele particulier.

Aux bandes de choc sont associ´ees les bandes d’absorption souvent remarquables dans les images optiques de galaxies barr´ees, qui manifestent la brutale augmentation de densit´edu gaz au passage des chocs. Athanassoula (1992b) a montr´e comment la forme de ces bandes de poussi`ere d´epend du potentiel et de l’existence des r´esonances internes de Lindblad. La pr´esence de ces derni`eres est n´ecessaire pour expliquer le d´eveloppement de bandes de choc d´ecal´ees par rapporta ` l’axe majeur du cˆot´e aval. Lorsque la concentration de masse centrale est trop petite ou que la vitesse angulaire de la barre est trop grande pour qu’une r´esonance interne de Lindblad existe, les trajectoires du gaz ont la mˆeme orientation que les orbites x1, et les bandes de choc sont soit confondues avec l’axe majeur de la barre, soit absentes. Par ailleurs, une barre faible induit des bandes de choc de forme courbe, tandis qu’une barre forte forme des bandes de choc rectilignes, telles que souvent observ´ees dans les galaxies barr´ees pr´ecoces.

Au passage des bandes de choc, le gaz change abruptement de direction : le cisaillement est tr`es important et le gaz, perdant une partie de son moment angulaire, coule vers les r´egions 26 centrales. Lorsqu’une r´esonance interne de Lindblad existe, le gaz s’accumule dans un anneau ; sinon, il peut ˆetre transf´er´epluspr`es des r´egions nucl´eaires. Cette accumulation de gaz peut ˆetre d’ampleur suffisante pour rendre le disque gazeux circumnucl´eaire auto-gravitant, qui deviendrait alors instable vis-`a-vis d’une onde spirale ou barr´ee sans que la composante stellaire centrale ne le soit. N´eanmoins, un m´ecanisme plus r´ealiste pour rendre compte des observations d’ondes de densit´enucl´eaires (dont la corotation est trouv´ee co¨ıncider avec la r´esonance interne de Lindblad de la barre principale) a ´et´epropos´e par Masset & Tagger (1997) : il s’agit d’un couplage non- lin´eaire entre les deux ondes, qui ´echangent du moment cin´etique pour se renforcer mutuellement. Comme la dispersion de vitesse des ´etoiles est tr`es grande dans la zone circumnucl´eaire, l’onde secondairesed´eveloppe plus facilement dans le gaz.

Une autre cons´equence des flux de gaz va nous int´eresser plus directement : le d´eclenchement de sursauts de formation d’´etoiles. La loi de Schmidt, qui relie de fa¸consimpleletauxde formation d’´etoiles `aladensit´e de gaz, semble valable aussi bien dans les disques de galaxies que dans leurs r´egions circumnucl´eaires ou dans les r´egions `a sursaut de formation d’´etoiles intense, au-del`a d’un certain seuil en densit´e du gaz (Kennicutt 1998b). Elle s’exprime, en termes de ∝ n n densit´es surfaciques, sous la forme ΣSFR Σgaz,o`u vaut de l’ordre de 1.4 . De l’application de cette loi, il r´esulte imm´ediatement que la formation d’´etoiles doit ˆetre particuli`erement intense au centre des galaxies barr´ees, et l’efficacit´e de formation d’´etoiles plus ´elev´ee qu’au centre des galaxies non barr´ees (puisque la loi de Schmidt est non-lin´eaire).

Regan et al. (1997), en ajustant un mod`ele hydrodynamique au champ de vitesse (observ´e dans la raie Hα) d’une galaxie fortement barr´ee, NGC 1530, estiment que le taux d’accr´etion −1 effectif sur la r´esonance interne de Lindblad est d’environ 1 M.an . Tout le gaz qui atteint le rayon de l’ILR, dans le prolongement de la bande de choc, n’y est pas pi´eg´e. En effet, la majeure partie du gaz repart en direction de la bande de choc oppos´ee, comme indiqu´e dans Fig. III.4. Cependant, on observe fr´equemment des taux de formation d’´etoiles centraux plus ´elev´es que cette estimation du taux d’accr´etion (par exemple dans NGC 5383, ´etudi´ee par Sheth et al. 2000). C’est un argument qui tenda ` favoriser, pour le d´eroulement de la formation d’´etoiles, des sursauts entrecoup´es de p´eriodes de latence, pendant lesquelles le disque ou l’anneau de gaz se remplit, plutˆot qu’une activit´econtinue.

III.4 Simulations des barres incluant le gaz et la formation d’´etoiles

L’´equipe de dynamique galactique de Gen`eve a construit une s´erie de simulations num´eriques ´evolutives des galaxies barr´ees, `a trois dimensions, prenant en comptea ` la fois le couplage gravi- tationnel entre les ´etoiles et le gaz interstellaire (Friedli & Benz 1993) et la r´egulation dynamique par la formation d’´etoiles (Friedli & Benz 1995). Les effets de la formation d’´etoiles incluent, en plus de la consommation de gaz, l’injection d’´energie thermique, d’´energie m´ecanique (par les ventsetlessupernovædetypeII),etl’´evolution chimique.

Les principaux r´esultats sont les suivants :

— Une barre gazeuse ne peut pas exister sans barre stellaire, la dynamique du gaz ´etant essentiellement dict´ee par le potentiel stellaire. Cependant, la barre gazeuse est toujours plus fine, et elle peut ˆetre “forte” mˆeme si elle est engendr´ee par une distorsion ovale. Par III. Les galaxies spirales barr´ees 27

cons´equent, les distorsions ovales fournissent au mˆeme titre que les barres fortes un moyen efficace de provoquer des ´ecoulements de gaz (voir en particulier, en illustration de ce fait, la galaxie NGC 4102, montr´ee dans le chapitre IV.3, et dont l’activit´e est discut´ee dans le chapitre VI).

— La vitesse de la barre d´ecroˆıt au cours du temps dans les syst`emes non collisionnels purs (c’est `a dire uniquement stellaires), mais se stabilise dans les syst`emes comportant une composante dissipative, d`es que suffisamment de masse a ´et´e accr´et´ee au centre. Ce fait peut donc empˆecher (si l’accr´etion de masse au centre n’est pas trop ´elev´ee) les r´esonances internes de Lindblad de se renforcer, ce qui est un m´ecanisme de destruction des barres.

—Lecrit`ere local de stabilit´e de Toomre contre l’effondrement gravitationnel (le gaz est stable si Q = vsκ > 1, o`u v est la vitesse du son, κ la fr´equence ´epicyclique et µ la densit´esur- g πGµg s g facique du gaz) reproduit qualitativement la distribution spatiale observ´ee de la formation d’´etoiles.

— La formation d’´etoiles est intense tout le long de la barre lorsque celle-ci s’est form´ee, dans un disque initialement axisym´etrique, il y a moins de 5.108 ans, mais confin´ee au centre et aux extr´emit´es de la barre lorsqu’elle est ´evolu´ee. Il n’est cependant pas clair si cette ´evolution s’applique aussi aux barres faibles ou non.

— Les gradients de m´etallicit´e du gaz et des ´etoiles sont r´eduits dans toute l’´etendue du disque (Friedli et al. 1994), et ce d’autant plus que la barre est plus forte. Cependant, la formation d’´etoiles intense dans les r´egions centrales est capable d’accentuer le gradient localement, sur une ´echelle de l’ordre du kpc.

— La formation d’´etoiles r´eduit la compacit´e de la concentration de masse centrale, par rapport au r´esultat des simulations n’incluant pas la conversion de gaz en ´etoiles. Par cons´equent, la possible destruction de la barre par l’accumulation de masse centrale, qui d´epend de sa compacit´e, est empˆech´ee ou retard´ee.

Un param`etre important pour l’instabilit´edynamique`al’´egard d’une barre est le rapport de masse du bulbe et du disque : un petit rapport favorise l’instabilit´eetentraˆıne des taux d’accr´etion plus grands. Par ailleurs, Friedli & Benz (1993) trouvent que le taux d’accr´etion de gaz au centre (int´egr´e au cours du temps) augmente si le rapport d’axes b/a de la barre (mesur´e `alamoiti´e du rayon de corotation) diminue. Cependant, ce r´esultat n’est pas enti`erement convaincant, car le rapport d’axes ´evolue au cours du temps, et un mod`ele parmi ceux explor´es est discordant.

Ces simulations permettent de suivre l’´evolution conjointe des caract´eristiques de la barre et de l’activit´e de formation d’´etoiles. Martinet & Friedli (1997) ont compar´elesr´esultats des mod`eles `a des observations, les taux de formation d’´etoiles ´etant estim´es `apartirdelacouleur infrarouge F25/F100, et la force de la barrea ` partir du rapport de sa longueura ` la taille du disque ou de son rapport d’axes b/a. Ces observations tendent `a montrer que c’est dans les galaxies fortement barr´ees que l’activit´e de formation d’´etoiles est la plus intense, mais que les deux quantit´es force de la barre – taux de formation d’´etoiles ne sont pas corr´el´ees. En effet, une fraction importante des galaxies fortement barr´ees ont une activit´e de formation stellaire similaire `a celle des galaxies faiblement barr´ees. Cette observation s’explique par le fait que 28 la formation d’´etoiles se produit sur une ´echelle de temps bien plus petite que l’´evolution du potentiel : le taux de formation d’´etoiles au centre n’augmente significativement qu’une fois que la densit´e de gaz a atteint un seuil critique, et le gaz est alors consomm´e`auntauxplusgrand −1 −1 que le taux d’accr´etion par l’effet de la barre (de l’ordre de 10 M.an contre 1 M.an ). La barre peut donc rester de grande amplitude longtemps apr`es qu’un sursaut se soit produit et ´evanoui.

Martin & Friedli (1997) ont par ailleurs compar´elespr´edictions des simulations `a la distri- bution observ´ee des r´egions HII dans les barres de galaxies tardives, ainsi qu’aux diff´erences morphologiques observ´ees entre les barres stellaires et les barres observ´ees en Hα.Danscer- taines barres suppos´ees de formation r´ecente, la formation d’´etoiles peut ˆetreintenselelongde la barre, comme dans NGC 7479, mais mˆeme dans cette galaxie, pour des valeurs raisonnables des param`etres qui gouvernent les conditions et les effets de la formation d’´etoiles, presque tout le gaz qui coule vers les r´egions centrales atteint celles-ci sans avoir ´et´econsomm´e.

III.5 Importance des barres pour l’´evolution des galaxies isol´ees

La redistributiona ` grande ´echelle du gaz interstellaire a d’autres cons´equences. Premi`erement, une anti-corr´elation a ´et´emiseen´evidence entre la force des barres et le gradient radial de m´etallicit´e des disques, ce qui est dˆu`a la migration du gaz de la partie interne du disque (`a l’ext´erieur de la corotation) vers la partie externe en direction de la r´esonance externe de Lind- blad, et de l’int´erieur de la corotation vers les r´egions centrales. Une barre forte est capable de redistribuer le gazal’´ ` echelle de tout le disque.

Le fait que les simulations num´eriques puissent former une barre dans un disque quasi- spontan´ement et d’autre part pr´evoient qu’une barre peut ˆetre dissoute par une accumulation de masse au centre, ce qui est une cons´equence mˆeme de l’existence d’une barre forte, sugg`ere de plus que toutes les galaxies spirales peuvent avoir travers´e au moins une phase barr´ee au cours de leur ´evolution et que ce ph´enom`ene peut se reproduire cycliquement. Sur la base de son gradient radial de m´etallicit´e, Martin & Belley (1997) proposent que dans la galaxie NGC 4736, qui poss`ede une distorsion ovale (Bosma et al. 1977), une barre autrefois forte ait ´et´e partiellement d´etruite.

Il est admis que les effets dynamiques d’une barre peuvent contribuerafairecroˆ ` ıtre les bulbes dans les galaxies spirales isol´ees. On peut voir deux effets distincts : – La barre cr´ee des r´esonances verticales, qui peuvent attirer un grand nombre d’´etoiles (nou- vellement form´ees par l’accr´etion de gaz ou pas) sur des orbites ayant des excursions verticales importantes (Pfenniger & Norman 1990). La structure verticale des r´egions internes des galax- ies barr´ees est ainsi tr`es caract´eristique, de forme rectangulaire (“boxy”) lorsque la barre est observ´ee align´ee avec la ligne de vis´ee, et en forme de cacahu`ete (“peanut”) lorsqu’elle est vue par la tranche (Combes et al. 1990). – Une augmentation importante de la concentration de masse au centre a pour effet de re- pousser `a un plus grand rayon la r´esonance interne de Lindblad, et aussi de la rendre plus forte, ce qui g´en`ere des orbites chaotiques qui perturbent alors les orbites principales de la famille x1.Parceph´enom`ene, les orbites qui sous-tendent la barre disparaissent et la barre III. Les galaxies spirales barr´ees 29 est d´etruite (Hasan & Norman 1990). Les ´etoiles qui faisaient la structure de la barre forment alors un bulbe ou viennent grossir un bulbe pr´eexistant. Il peut subsister une d´eformation tri- axiale dans ce bulbe, ce qui serait un signe caract´eristique de l’existence pass´ee d’une barre (Pfenniger & Norman 1990).

Il existe donc une possibilit´e de former partiellement les bulbes apr`es les disques, dans des galaxies n’ayant pas subi de fusion avec un compagnon, mˆeme si les ´etoiles qui se retrouvent dans le bulbe sont aussi vieilles que les ´etoiles du disque. Cela implique une comp´etition avec le sc´enario de formation hi´erarchique des galaxies, car l’´evolution morphologique se fait dans le premier cas `a partir d’une seule galaxie (accr´etant ´eventuellement du gaz) et non de la fusion de plusieurs galaxies. Une telle ´evolution induite par une barre est cependant beaucoup plus lente et conserve le disque. 30 31

Chapitre IV Echantillon de galaxies

IV.1 Programmes d’observation

L’´echantillon utilis´ea´et´e construit en plusieurs ´etapes. Le programme d’observations ISO- CAM qui a sous-tendu le sujet de la th`ese est Cambarre, qui avait pour objet de cartographier quelques galaxies spirales barr´ees proches et bien r´esolues, dans le but de caract´eriser leur activit´e de formation d’´etoiles et sa distribution spatiale. Les 13 galaxies s´electionn´ees sont brillantes en infrarouge lointain et de types morphologiques vari´es, toutes faiblement inclin´ees sur la ligne de vis´ee et `a des distances comprises entre 11 et 38 Mpc, ce qui garantit, vu leurs tailles, que beaucoup de d´etails sont r´esolus. Leurs magnitudes absolues dans le bleu varient entre −19.25 et −20.84, c’est `a dire qu’elles moins brillantes que la magnitude typique de la fonction de lu- minosit´edeSchechter(−21). Toutes sont fortement barr´ees, `a l’exception de NGC 4535 qui a une barre faible et qui est situ´ee `alap´eriph´erie sud de l’amas de Virgo.

Ces galaxies ont ´et´e cartographi´ees dans deux filtres larges : LW2, centr´e`a7µm (5–8.5 µm), et LW3 centr´e`a15µm (12–18 µm). Ces filtres ont ´et´e choisis, le premier parce qu’il recouvre un massif de bandes d’´emission aromatiques, et le deuxi`eme parce qu’il ´etait suppos´ecou- vrir le continuum thermique des petits grains de poussi`ere classiques, avec une contamination n´egligeable par les bandes aromatiques. La s´eparation non ambigu¨e de ces deux phases de poussi`ere aurait ´et´e possible grˆace `a des observations en mode de spectro-imagerie d’ISOCAM, mais qui auraient n´ecessit´e un temps d’observation r´edhibitoirement long, et au prix d’une bien moins grande sensibilit´e. Nous verrons que les filtres choisis sont bien adapt´es `a cette ´etude, mˆeme si l’interpr´etation de l’´emission qu’ils collectent a m´enag´e quelques surprises. Pour v´erifier sur quelques exemples que les filtres larges permettent bien de s´eparer les contributions des diff´erentes phases ´emettrices, et pour obtenir des informations plus fines sur leur distribu- tion spectrale d’´energie et leurs variations spatiales `a l’int´erieur des galaxies, des spectres ont ´egalement ´et´e obtenus, avec les filtres variables circulaires (CVF) d’ISOCAM, pour deux galaxies de cet ´echantillon.

Dans le but d’augmenter la taille de l’´echantillon et de disposer d’un groupe de galaxies de r´ef´erence, non barr´ees ou dont la barre est de force interm´ediaire, deux autres programmes ont ´et´e utilis´es, ainsi que pr´evu par l’argumentaire de Cambarre : – Camspir, dont les cibles sont six galaxies spirales de grande taille angulaire, certaines in- trins`equement g´eantes (diam`etres optiques entre 11 et 29, qui se traduisent par des tailles 32 physiques entre 14 et 60 kpc). Ces galaxies permettent une analyse spatiale tr`es d´etaill´ee `a la r´esolution angulaire d’ISOCAM. Trois d’entre elles ont ´egalement ´et´eobserv´ees en mode de spectro-imagerie. – Virgo, dont le but ´etait de caract´eriser les propri´et´es en infrarouge moyen d’un ´echantillon complet de galaxies de l’amas proche Virgo. Je n’en ai utilis´e qu’une partie, c’estadireles ` galaxies spirales, barr´ees ou non, suffisamment r´esolues pour leur appliquer une d´ecomposition ´el´ementaire en r´egions centrales et disque (voir l’Atlas et le chapitre VI).

Enfin, pour disposer d’une meilleure statistique, j’ai compl´et´ecet´echantillon avec des galaxies selectionn´ees dans l’archive publique d’ISO, qui appartiennenta ` deux autres programmes : – Sf glx (PI G. Helou), qui contient un tr`es grand nombre de galaxies, d´efini dans le but g´en´eral de mieux cerner les propri´et´es du milieu interstellaire et les processus de formation d’´etoiles. – Irgal (PI T. Onaka), en pr´eparation et compl´ement `a la future mission spatiale infrarouge IRTS, par le Japon. J’ai choisi parmi ces galaxies celles qui n’abritent pas de noyau de Seyfert et dont le disque stellaire ne pr´esente pas de forte distorsion par des interactions gravitationnelles. Notons que M 51 et M 101, par exemple, qui font partie du projet Camspir, sont bien connues pour subir des interactions de mar´ee, visibles dans la morphologie externe de leurs bras spiraux. Cependant, les d´eformations induites restent mod´er´ees, et les cons´equences dynamiques affectent surtout le disque (en particulier dans la structuration des bras), et beaucoup moins les r´egions centrales. Par ailleurs, il n’existe pas de galaxie qui soit id´ealement isol´ee de tout le reste de l’univers. C’est pourquoi des interactions mod´er´ees ont ´et´etol´er´ees dans la s´election des galaxies de Sf glx et Irgal.

La liste compl`ete des galaxies de l’´echantillon, avec leurs caract´eristiques g´en´erales (coor- donn´ees, distance, type morphologique, magnitude bleue, diam`etre optique et type nucl´eaire) se trouve dans l’article inclus dans le chapitre VI.

IV.2 Publication : Atlas

An atlas of mid-infrared dust emission in spiral galaxies H. Roussel, L. Vigroux, A. Bosma, M. Sauvage, C. Bonoli, P. Gallais, T. Hawarden, J. Lequeux, S. Madden & P. Mazzei Astronomy & Astrophysics 369, 473-509 (2001)

Voici le r´esum´e de cet article :

Nous avons pr´esent´e les cartes compl`etes de l’´emission de la poussi`ere `a7µm de 43 galaxies de types morphologiques compris entre S0/a et Sdm,a ` une r´esolution angulaire plus petite que 10 et une sensibilit´e de l’ordre de 5 µJy arcsec−2. Nous avons aussi d´ecrit en d´etail la r´eduction des donn´ees, fourni les flux totauxa7et15 ` µm et soulign´elespropri´et´es morphologiques g´en´eriques de cet ´echantillon. Comme il existe une litt´erature fournie sur beaucoup de ces galaxies ´etudi´ees individuellement, nous nous sommes restreints aux caract´eristiques communes. La description d´etaill´ee de l’´echantillon et l’interpr´etation des donn´ees figurent ensemble dans l’article I.

La morphologie est tr`es semblable en infrarouge moyen et en optique,a ` l’exception des IV. Echantillon de galaxies 33 galaxies particuli`eres dont l’´emission de la poussi`ere est fortement concentr´ee : l’´emission `a7et 15 µmsuitdepr`es les bras spiraux, les lentilles et les complexes g´eants individuels. Cependant, elle apparaˆıt moins ´etendue que l’´emission stellaire et tend `aser´eduire dans les bras spiraux lisses et non structur´es, frequ´emment observ´es dans les disques externes des galaxies pr´ecoces, et particuli`erement des galaxies d´eficientes en gaz HI. Cela peut se relier `a des faibles densit´es de surfacea ` la fois dans le gaz et dans les populations stellaires jeunes. Une autre diff´erence frappante concerne l’aspect de la barre, o`ulesr´eponses des traceurs du milieu interstellaire et des ´etoiles au potentiel gravitationnel sont clairement distinctes. Enfin, les r´egions centrales se distinguent dans nos cartes comme des entit´es aux propri´et´es particuli`eres, et seront ´etudi´ees en d´etail dans l’article I.

La caract´eristique principale des couleurs en infrarouge moyen (les rapports des fluxa15et ` 7 µm) est leur constance remarquable `a travers les disques, except´edansdesr´egions circum- nucl´eaires et dans quelques complexes brillants des bras spiraux (dont le meilleur exemple se trouve dans les bras externes en interaction de mar´ee de M 101). 34 A&A 369, 473–509 (2001) Astronomy DOI: 10.1051/0004-6361:20010154 & c ESO 2001 Astrophysics

An atlas of mid-infrared dust emission in spiral galaxies

H. Roussel1,L.Vigroux1,A.Bosma2, M. Sauvage1, C. Bonoli3, P. Gallais1,T.Hawarden4, J. Lequeux5, S. Madden1, and P. Mazzei3

1 DAPNIA/Service d’Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France 2 Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 4, France 3 Osservatorio Astronomico di Padova, 5 Vicolo dell’Osservatorio, 35122 Padova, Italy 4 Joint Astronomy Center, 660 N. A’ohoku Place, Hilo, Hawaii 96720, USA 5 Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France Received 3 November 2000 / Accepted 23 January 2001 Abstract. We present maps of dust emission at 7 µmand15µm/7 µm intensity ratios of selected regions in 43 spiral galaxies observed with ISOCAM. This atlas is a complement to studies based on these observations, dealing with star formation in centers of barred galaxies and in spiral disks. It is accompanied by a detailed description of data reduction and an inventory of generic morphological properties in groups defined according to bar strength and HI gas content.

Key words. atlas – galaxies: spiral – galaxies: ISM – infrared: ISM: continuum – infrared: ISM: lines and bands

1. Introduction The general description of the sample and detailed photometric results are given in Paper I, where the spectra The mid-infrared maps presented here are part of a larger are also published. Here, we provide the details of obser- sample of nearby spiral galaxies analyzed in Roussel et al. vations and data reduction, total fluxes at 7 and 15 µm, (2001a and b: Papers I and II). Paper I primarily in- a morphological description of galaxies grouped into ap- vestigates the dynamical effects of bars on circumnuclear propriate categories and maps at 7 µm, along with optical star formation activity, studied through dust emission at images. The 15 µm maps have very similar morphology 7and15µm, and proposes an interpretation of these mid- and are therefore not shown. These images demonstrate infrared data in conjunction with information on molec- the peculiar character of circumnuclear regions seen in the ular gas and stellar populations. Paper II deals with the mid-infrared. Information on the F15/F7 colors of bright use of mid-infrared fluxes as a star formation indicator complexes and other selected regions is added. in spiral disks. Observations belong to guaranteed time All of the maps are published for the first time, ex- programs of ISOCAM, a camera operating between 5 and cept those of M 51 (Sauvage et al. 1996) and NGC 6946 18 µm onboard the satellite ISO (described by Cesarsky (Malhotra et al. 1996), which are both re-analyzed here. et al. 1996). The sample consists of a first group of large and regular spirals, mainly barred, and of a second group 2. Observations of spiral galaxies belonging to the Virgo cluster. The an- All galaxies were observed with two broadband filters, gular resolution of ≈10 (HPBW), combined with spec- troscopic information from the same instrument, enables a LW3 centered at 15 µm (12–18 µm)andLW2centeredat 7 µm (5–8.5 µm), that we shall hereafter designate by their detailed view of the variations of two dust phases (uniden- central wavelength. Maps covering the whole infrared- tified infrared band carriers and very small grains, as char- acterized by e.g. D´esert et al. 1990), tracing different phys- emitting disk were built in raster mode, with sufficient overlap between adjacent pointings (half the detector field ical conditions. A stellar contribution can also exist at 7 µm (Boselli et al. 1998), but is negligible except in a few of view in most cases) to avoid border artifacts and to pro- early-type galaxies. vide redundancy for spoiled exposures. In all cases, the field of view is large enough to obtain a reliable determi- nation of the background level, except for NGC 4736 and [email protected] 6744. NGC 5457, the largest spiral in the sample with an Send offprint requests to: H. Roussel, e-mail: Based on observations with ISO, an ESA project with in- optical size of nearly 30 , was imaged with a combination struments funded by ESA Member States (especially the PI of two overlapping observations made during different rev- countries: France, Germany, The Netherlands and the UK) and olutions of the satellite. The pixel size is either 3 or 6 , with the participation of ISAS and NASA. depending on the galaxy size, but for Virgo galaxies it 474 H. Roussel et al.: An atlas of MIR dust emission in spirals was always 6, in order to increase thesignaltonoisera- 2 – If needed, we mask out the borders of the detector tio. The half-power/half-maximum diameters of the point which are not sufficiently illuminated to allow a proper spread function are respectively 6.8/3.1 at 7 µmwith flat-fielding; a3 pixel size, 9.5/5.7 at 7 µmwitha6 pixel size, 3 – The removal of cosmic ray impacts is complicated by 9.6/3.5 at 15 µmwitha3 pixel size and 14.2/6.1 at the long-duration memory effects they can produce. The 15 µmwitha6 pixel size. Table 1 summarizes the rele- arrival of an energetic cosmic ray may be followed by ei- vant information regarding the observations. ther a slowly decreasing tail or a responsivity drop, as To estimate the relative importance of different emit- explained above. The vast majority of glitches are, how- ting species in different galactic regions, spectral imaging ever, of short duration (one to three exposures) and can between 5 and 16 µm of the inner disks of five bright galax- be rejected by median filtering. To also remove small tails ies was also carried out. These observations are essential and plateaus induced by glitch pile-up, the exposures im- to interpret broadband imaging results. They consist of mediately surrounding exposures flagged by the deglitcher one pointing with two circular variable filters, from 16.5 were examined and their flux level compared with that of to 9 µm and from 9.6 to 5 µm, with a spectral resolution adjacent non-flagged exposures, i.e. we iterated the me- λ/∆λ(FWHM) = 35 to 50. The sampling varies between dian filtering once after rejecting the cosmic ray peaks. 0.24 and 0.45 of ∆λ(FWHM). This filtering cannot blindly be applied everywhere. First, at a step between a faint and a bright illumination level and vice-versa, some real signal variation may be masked 3. Data analysis by the deglitcher. Therefore, the signal in the first and last 3.1. Broadband filter maps exposures for each sky position was compared with that in the neighboring exposures for the same sky position, Data reduction was performed using and adapting the and flags cancelled if necessary. Second, during a point- ISOCAM Interactive Analysis package (CIA). The main ing on a bright peaked source such as galactic circumnu- difficulty and source of uncertainty is the slow time re- clear regions, the noise includes, in addition to the readout sponse of the detector, which is typical of cold photo- and photon noises, high amplitude fluctuations which are conductors. Below 20 K, the mobility of carriers becomes approximately proportional to the signal and due to the very slow, which generates long time constants, both in satellite jitter. For the purpose of glitch detection, an ad- the bulk of the photoconductor and at contact electrodes. ditional noise component proportional to the signal was The typical time spent per pointing is ≈40–100 s whereas thus tuned to reproduce the jitter effects, and for very in general several hundred seconds are needed to reach bright sources, defined by a flux threshold set above the stabilization within 10% of the flux step. background, fluctuations of 20% around the median flux Several causes of memory effects can be distinguished, were allowed. The memory effects following some glitches, although they always involve the same physics of the de- for which no model exists, are temporarily set aside and tector. We will use different designations for convenience: examined at step 6; – (short-term) transients: they are due to the slow stabi- 4 – Short-term transients are corrected for by using the lization following a flux step (either upward or downward), latest available technique taking into account the detector after moving from one sky position to another; they are characteristics (Coulais & Abergel 2000). The parameters the origin of remnant images seen after observing a bright of the model have been determined in low and uniform source; illumination cases only, but are likely to vary with signal – long-term transients: drifts and oscillations visible dur- intensity and are inexact in the presence of strong spa- ing the whole observation and influenced by previous his- tial gradients. Indeed, when switching from a moderate tory; contrary to short-term transients, no model exists to flux level to a bright resolved source, the correction hardly correct for them; modifies the data. After a large amplitude flux step, the – glitch tails: slow return to the sky flux level following stabilization is, however, faster, and thus the last expo- the deposition of energy in the detector by intense cosmic sures are likely closer to the real flux than in the case of rays (this is an unpredictable effect); small flux steps. The readout histories of pixels imaging – responsivity drops: they also follow cosmic ray impacts galactic nuclei or extranuclear bright complexes were care- and produce “holes” which can last more than one hun- fully examined after step 7 to check whether the computed dred exposures (this is again an unpredictable effect). flux level was consistent with the level expected from the The data reduction proceeded in the following way: last valid exposures; if not, it was replaced with the mean 1 – The subtraction of the dark current follows a model signal in the last part of the readout history that was ap- which predicts its time evolution for even and odd rows of proximately flat as a function of time. Furthermore, after the detector. It accounts for variations along each satellite 1 seeing a bright source, remnant images are not completely orbit and along the instrument lifetime ; suppressed and can persist for several successive pointings; 1 It is described in “The ISOCAM dark current calibra- they are masked out at step 9; tion report” by Biviano et al. (1998), which can be found at: 5 – As long-term transients are not taken into account http://www.iso.vilspa.esa.es/users/expl lib/CAM list. by the above correction, they were approximated and re- html moved as one offset for each sky position, with respect to H. Roussel et al.: An atlas of MIR dust emission in spirals 475

Table 1. Raster observations. Listed are the number of the figure showing the maps, the project name (B: Cambarre;S:Camspir; F: Sf glx;V:Virgo), the pixel size, the number of pointings projected onto the raster map, the field of view, the number of exposures per pointing, the integration time per exposure, total on-target times at 15 and 7 µm, and total fluxes. When two figures are given, the first one corresponds to 15 µm and the second one to 7 µm. For NGC 6946, observed with 6 pixels, each pointing is displaced from the previous by a fractional number of pixels, so that the pixel size of the projected map is 3

a name Fig. proj. pix. Np map size nexp tint T15 T7 F15 F7 ()()(s)(s)(mJy) N289 6c B 3 16 240 26 5.0 2086. 2086. 327.8 ± 25.6 342.9 ± 14.7 N337 7b B 3 16 240 26 5.0 2131. 2137. 297.9 ± 24.0 336.1 ± 17.9 N613 5c B 3 25 288 25 5.0 3170. 3170. 1566.5 ± 104.0 1473.3 ± 71.4 N1022 7a B 3 16 240 26 5.0 2071. 2076. 802.3 ± 86.4 444.4 ± 45.3 N1097 5d B 3 25 288 25 5.0 3119. 3114. 2269.2 ± 167.4 2128.6 ± 125.4 N1365 5d S 6 16 480 61 2.1 1978. 1984. 4436.7 ± 764.5 3691.9 ± 616.6 N1433 5b B 3 25 288 25 5.0 3165. 3175. 355.3 ± 41.0 381.3 ± 33.8 N1530 5b B 3 16 240 26 5.0 2137. 2126. 606.1 ± 39.2 573.9 ± 39.1 N1672 5c B 3 25 288 25 5.0 3170. 3155. 2020.5 ± 123.0 1985.0 ± 129.2 N4027 7b B 6 9 384 41, 42 2.1 787. 787. 676.7 ± 95.5 775.8 ± 68.2 3 4 132 42, 41 2.1 350. 359. N4535 6b B 6 16 480 27, 20 5.0 2066. 2056. 1127.9 ± 181.4 1136.6 ± 68.9 3 4 132 26 5.0 539. 534. N4691 7a B 6 4 288 41, 42 2.1 359. 357. 795.9 ± 185.6 613.5 ± 83.1 3 4 132 42, 41 2.1 352. 354. N4736 (M94) 6d S 6 9 288 20 5.0 917. 922. 4204.5 ± 240.6 3913.9 ± 225.8 N5194 (M51) 6d S 3 100 663 27 2.1 5384. 5390. 8003.2 ± 493.5 8598.7 ± 552.1 N5236 (M83) 6a S 6 49 768 21 5.0 4918. 5155. 20098.4 ± 803.7 18474.9 ± 899.7 N5383 5a B 6 9 384 54 2.1 1024. 1022. 332.6 ± 61.9 350.2 ± 62.1 3 4 156 54, 52 2.1, 5.0 459. 1083. N5457 (M101) main † 6c S 6 49 | 1302 20, 21 5.0 4949. 4924. 5424.3 ± 322.0 6034.0 ± 116.7 Earm† 625|×972 25 5.0 3155. 3140. N6744 6b S 6 16 480 25 5.0 2031. 2041. 1497.4 ± 125.7 2419.4 ± 52.3 N6946 6a F 6-3 64 759 13, 9 2.1, 5.0 1585. 2872. 10651.6 ± 1767.2 11648.8 ± 678.6 N7552 5a B 6 16 480 26 5.0 2091. 2112. 3 9 168 26 5.0 1184. 1174. 2767.6 ± 193.7 1826.2 ± 168.5 Virgo cluster sample:

N4178 8a V 6 25 576 17, 18 2.1 819. 860. 181.5 ± 48.0 228.5 ± 24.6 N4192 8a V 6 49 768 17, 18 2.1 1528. 1625. 630.0 ± 99.6 900.8 ± 68.3 N4293 9a V 6 25 576 17, 18 2.1 804. 844. 188.6 ± 42.8 159.5 ± 25.3 N4351 8b V 6 9 384 17, 15 2.1 291. 283. 45.6 ± 26.352.6 ± 8.7 N4388 9a V 6 25 576 17, 18 2.1 812. 863. 1008.2 ± 244.0 499.4 ± 77.8 N4394 9b V 6 16 480 16 2.1 480. 478. 139.0 ± 41.0 161.2 ± 19.1 N4413 9b V 6 9 384 17, 16 2.1 302. 281. 93.0 ± 31.489.3 ± 11.0 N4430 8b V 6 9 384 17, 16 2.1 294. 279. 98.0 ± 23.5 132.5 ± 13.9 N4438 9c V 6 49 768 17, 18 2.1 1528. 1629. 209.1 ± 34.7 231.9 ± 26.9 N4450 9c V 6 25 576 17 2.1 791. 844. 169.7 ± 42.5 185.1 ± 14.6 N4491 9d V 6 9 384 17, 15 2.1 310. 287. 81.1 ± 25.230.5 ± 7.6 N4498 9d V 6 16 480 16 2.1 493. 478. 94.6 ± 19.2 112.9 ± 11.8 N4506 9e V 6 9 384 17, 16 2.1 294. 277. 12.7 ± 5.221.1 ± 9.8 N4567/ 8c V 6 25 576 17, 18 2.1 779. 829. 293.4 ± 15.5 317.9 ± 16.4 N4568 1099.0 ± 127.6 1074.7 ± 64.8 N4569 9e V 6 64 864 16, 18 2.1 1864. 2108. 939.3 ± 125.1 843.5 ± 54.1 N4579 9f V 6 36 672 17, 19 2.1 1161. 1306. 619.2 ± 85.1 672.5 ± 37.5 N4580 9f V 6 9 384 17, 16 2.1 298. 279. 103.9 ± 24.2 102.6 ± 7.7 N4633/ 8c/ V 6 36 672 17, 19 2.1 1123. 1266. 30.0 ± 9.530.3 ± 9.1 N4634 8d 258.2 ± 40.7 278.3 ± 35.0 N4647 9g V 6 36 672 17, 18 2.1 1127. 1310. 472.3 ± 32.0 474.3 ± 17.2 N4654 8d V 6 25 576 17, 18 2.1 814. 852. 1018.6 ± 78.4 1049.4 ± 42.9 N4689 9g V 6 25 576 17, 18 2.1 787. 863. 329.7 ± 37.4 340.9 ± 16.3 a This is the minimum number of exposures. At the beginning of the observation for each filter, it is in general much larger, in order to get closer to stabilization. † Themapsizegivenisthatofthecombinationofthemainmapwiththeeasternarmmap. 476 H. Roussel et al.: An atlas of MIR dust emission in spirals the last sky position taken as a reference, step by step Table 2. Parameters of the spectral observations. They include backwards and assuming that all pixels follow the same the pixel size, the ratio of the field of view to that of raster evolution. This is done by comparing the median of the maps, the number of exposures per wavelength, the integration highest possible number of pixels seeing the off-source sky time per exposure and the total useful time in the current pointing and in any of the already-corrected n t T pointings, and selecting the same set of pixels in both, name pix. size ( ) frac. size exp int (s) tot (s) a since at this stage the images have not been flat-fielded. N613 3 0.33 12 2.1 3960. As it is treated as an offset, the correction does not mod- N1097b 6 0.66 9–10 2.1 3276. ify the source flux but flattens the sky and allows better N1365 6 0.40 15 2.1 4918. determination of the background level. It is similar to the N5194 6 0.29 15 2.1 4918. long-term transient correction in the SLICE software of N5236 6 0.25 15 2.1 4918. Miville-Deschˆenes et al. (2000); a 6 – Short-term transients after a strong downward flux At 3 , no calibration flat-field in narrow filters was available, step which were imperfectly corrected at step 4, or glitch so we did not apply the division by fffilter (see last paragraph tails, left aside at step 3, and with the strongest time of Sect. 3.2). b µ derivatives (hence the easiest to detect), were masked out At the change of variable filter at 9.2 m, a downward jump by an automatic procedure. Drops below the background was visible in the whole field; to cancel it, we applied a unique offset to the second part of the spectrum. level, due to a responsivity drop after a glitch, were also masked out (without the need for strong time derivatives to design an automatic rejection); 3.2. Spectral imaging 7 – The valid exposures were averaged for each sky posi- tion; In order to be able to decompose broadband observations 8 – The flat-field response was systematically determined according to the various emitting species, low-resolution spectra between 5 and 16 µm were also obtained toward for each pixel as its mean value when illuminated by the background emission (masking the source), and then nor- the inner 3 × 3 or 1.5 × 1.5 of five galaxies. Table 2 malized by the mean in the 12 × 12 central pixels. When summarizes the parameters of these observations. for a given pixel the number of useable values is too low For spectral imaging, the principles of data reduction (which happens if the pixel mostly images the source, or are the same as for broadband filters. Additional problems is often rejected due to glitches or memory effects), or encountered in this type of observation are the following: when the error on the mean (computed from the disper- – The observation is split into two slightly overlapping sion) is higher than 5%, the flat-field response of this pixel parts: from 16.5 to 9.2 µm and then from 9.5 to 5 µm. The is not computed as the mean, but interpolated using the junction between the two circular variable filters, where neighboring pixels and the default calibration flat-field, the spectral transmission is low, can produce artefacts before applying the normalization. This method produces shortward of 9.2 µm for a few wavelength steps, especially better results than the use of raw calibration files. These because of non-stabilization; calibration data were employed only for NGC 4736 and – There is a small displacement of the source from one NGC 6744, for which the field of view is too small to ap- wavelength to another, the total shift reaching up to two ply the above method (the background is hardly seen). For 3 -pixels, which is due to the rotation and change of the Virgo galaxies, which are small compared to the camera circular variable filters. This effect was corrected for by fit- field of view and were mapped with numerous pointings, a ting the shape of the galactic central regions by a Gaussian time-variable flat-field was estimated, improving the final to measure their displacement on the array (for M 51, it quality of the maps. The correction for long-term drifts was not possible and was done manually at the change (step 5) was then iterated once for all galaxies (this is between the two segments of the variable filter); again similar to the SLICE processing); – Bright sources, such as galactic central regions, can cre- 9 – The most conspicuous residual memory effects escap- ate ghost images, due to reflections inside the instrument; ing rejection because they vary too slowly were removed in the case of NGC 1365, we could identify and isolate manually in each sky position image. They were identified regions contaminated by obvious stray light ghosts. The visually by comparing structure seen at the same sky coor- flux collected by ghosts in a uniform illumination case 2 dinates in different overlapping pointings (corresponding is ≈20% (ISOCAM handbook) . For resolved sources, it to different sets of pixels with different histories), taking depends on their position on the detector; advantage of the spatial redundancy of the observations; – The flat-field response used ignores any time or wave- 10 – The final map was built by projecting together all length dependency. Nevertheless, this source of error is the individual images on a sky rectangular grid; negligible compared with memory effects and the error on 11 – The conversion from electronic units to spectral en- the determination of the background level; ergy density units is made using the standard in-flight 2 ISO handbook volume III: CAM – the ISO camera, calibration data base. Siebenmorgen et al. 2000, SAI-99-057. It can be found at: http://www.iso.vilspa.esa.es/manuals/HANDBOOK/III/ cam hb/ H. Roussel et al.: An atlas of MIR dust emission in spirals 477

rising plateau from 11 to 16 µm: see Fig. 1 in Paper I), the spectral shape of the disk emission is well determined; – Because of ghosts, the flat-field response was not han- dled as in broadband filter observations. The observed sig- nal is the sum of the source, the background and their respective ghosts, multiplied by the flat-field response: I =(S + b + g(S)+g(b)) × ff. However, the calibra- tion flat-fields are also contaminated by ghosts: ffcal = ff × (1 + g(b)/b). Therefore, we define the corrected sig- nal as: Icorr =(I − zodi × ffcal)/fffilter, where zodi is the zodiacal spectrum estimated as described above and fffilter is a flat-field response measured with narrow fil- ters, which does not contain any ghost. We then obtain Icorr = S + g(S): only ghosts due to the source remain. The spectra resulting from the data processing de- scribed above are shown in Paper I (for central regions, disks, and the zodiacal emission), where they are dis- Fig. 1. Spectra of the central regions of the five galaxies ob- cussed. In Fig. 1 we reproduce the spectra of the galactic served in spectral imaging mode. They have been shifted for clarity by respectively 0.5, 1.3, 2.1 and 2.9 mJy arcsec−2 and central regions, with their noise and uncertainty on the multiplied by the numbers indicated on the right. Error bars zodiacal foreground (but no estimation of the errors due include the uncertainty on the zodiacal foregroung level, which to memory effects was possible). Contribution from stray- is small with respect to the central source brightness light ghosts can be of the order of 10–20% (see Sect. 3.5).

3.3. Photometry – Probably due to an inaccurate dark current subtraction amplified by the transient correction algorithm, the short- In some observations, the background is high compared wavelength spectrum can be negative at faint signal levels; to the source surface brightness, and it can be affected by – As sources occupy the whole field of view and no spectra inhomogeneities largely due to an incomplete correction of empty regions were taken during the observations, the for both short-term and long-term memory effects. The zodiacal foreground produced by interplanetary dust can- largest possible number of pixels has to be used, there- not be directly measured. We estimated it in the following fore, for a proper determination. In some other cases, a way: we use a high signal to noise ratio calibration obser- faint-level emission in spiral arms can be traced out to the vation,from16to5µm and then back from 5 to 16 µm map edge and likely continues outside the field of view. (which is useful to assess memory effects), of an empty However, this emission is extremely faint and does not 3 part of the sky . We apply an offset and a normalization introduce any significant error in integrated fluxes. The to this zodiacal spectrum in order to obtain the maximum only exceptions are NGC 4736 and NGC 6744, which are lower envelope to the average spectrum of the faintest pix- larger than the observation fields of view. For NGC 6744, els in our observation. The latter spectrum is composed 12% of the map in the southern part is blank, because the of zodiacal light with superimposed faint emission bands telescope did not move during 3 of the 16 programmed (UIBs: see Paper I). Then, two bracketing zodiacal spec- pointings. The given total fluxes are therefore lower lim- tra with the same normalization are placed symmetrically; its. Other uncertain fluxes are those of the apparent they are constrained to remain inside the dispersion lim- pair NGC 4567/68, which has slightly overlapping disks in its of the observed spectrum, at least where it is not con- projection. taminated by UIBs. The minimum and maximum zodia- For all galaxies, the integrated flux was measured in cal spectra estimated in this way are subtracted from the concentric circles and traced as a function of the number galaxy spectrum, which produces an upper and a lower of pixels, after excluding some sectors of the map where limit to the true spectrum. The minimum spectrum of the the galaxy extends to large distances, so as to see as much galaxy disk is also required to be positive, which tightens uncontaminated background as possible. The background the bracketing of the zodiacal spectrum. was fitted in the linear part of this curve. Due to the fact that the zodiacal light is several times Total fluxes are given in Table 1 and other photometric higher than the disk spectrum (outside the bright cen- results (see below) in Paper I. tral regions), the absolute intensity of the disk emission is subject to large uncertainties. However, since the zodi- acal light spectrum is mainly featureless (it consists of a 3.4. Separation of central regions from disks regularly rising part between 5 and 12 µmandaslowly A common feature of dust emission distribution in our 3 The identification number of this observation, “TDT galaxies is a very strong central peak in a region of typi- number” in the ISO archive, is: 06600401. cally 1 to 3 kpc. Sub-structures are barely seen because the 478 H. Roussel et al.: An atlas of MIR dust emission in spirals angular resolution is of the order of the size of these central – the deviation from stabilization, grossly estimated regions, but can sometimes be suspected from variations from the dynamics of each pixel’s response during the in the infrared color F15/F7. We measured fluxes from time spent on each pointing, as half the difference be- this central condensation after defining its radius from the tween the smoothed variation amplitude and the 99% 7 µm map. The galaxy center was first determined by fit- confidence level readout and photon noise. This error ting a two-dimensional Gaussian on an appropriate zone component is added linearly over all the pixels imaging whenever possible. Otherwise, it was placed by visual cri- the galaxy, because memory effects cannot be assimi- teria (for three galaxies). We computed the azimuthally lated to white noise. averaged surface brightness profile in elliptical annuli to compensate for the inclination on the sky, using as far The latter error is usually dominant, except for the faintest as possible orientation parameters derived from detailed galaxies. We obtain typical errors of ≈10% at 7 µmand kinematical analyses, otherwise from the position angle 18% at 15 µm (most galaxies having errors less than, re- and axis ratio of outer isophotes. This profile was decom- spectively, 18 and 30%). These do not take into account posed into a central region represented by a Gaussian and flux density calibration uncertainties, which are of the or- an inner disk represented by an exponential. The scale pa- der of 5%, with an additional 5% variation along each rameters were fitted on parts of the profile devoid of any orbit (ISOCAM handbook). obvious structure, such as rings or spiral arms, and each The five galaxies observed in spectral imaging mode component was truncated at the radius where the expo- provide the opportunity to perform checks on the agree- nential equals the Gaussian, which defined the photomet- ment between fluxes derived from independent observa- ric aperture radius for the circumnuclear region, RCNR. tions. Since the wavelength coverage of our spectra (5 to Dilution effects were corrected for in two ways. The about 16 µm) does not contain the whole 15 µm band (12– first consists of approximating central regions by point 18 µm), our comparison is limited to the 7 µm band. We sources and dividing the fluxes measured inside RCNR by simulated observations in the LW2 band (5–8.5 µm) us- the integral of the point spread function (PSF) inside the ing its quantum efficiency curve. The comparison with same aperture. The second one uses an iterative proce- the photometry performed on the 7 µmmapsisshown dure. The image is first re-sampled on a fine grid (replacing in Fig. 2. The agreement is perfect for NGC 1365, but each pixel by smaller pixels all taking the same value). The the spectra of the other four galaxies systematically over- brightest pixel of the original sampling is looked for, the estimate 7 µm fluxes with respect to raster maps. The PSF centered on it is subtracted from both images with a amplitude of the deviation (23% for NGC 613, 16% for small gain to ensure convergence of the process (5% here), NGC 1097, 12% for NGC 5194 and 17% for NGC 5236) and is then replaced in the fine-grid image with a Gaussian is of the order of what can be expected from stray light containing the same flux as the removed PSF, of FWHM ghosts from the circumnuclear regions, which would be about half the pixel size. This procedure is analogous to superimposed onto the source. This is consistent with the the CLEAN algorithm used in radio astronomy (H¨ogbom fact that an obvious ghost can be identified (outside the 1974). This is repeated until the residual image becomes central regions) only in the case of NGC 1365, whose cir- reasonably uniform. For central regions not much more cumnuclear region is thus likely free from the ghost it extended than the PSF, both estimates give equal results generates. It should then be remembered that photom- to a few percent; they differ in cases of a distribution flat- etry performed on raster maps is more reliable. See also ter than the PSF or in the presence of sub-structure, such F¨orster-Scheiber et al. (2001) for a photometric compari- as a ring. son between ISOCAM and ISOSWS spectra. Fluxes attributable to the pure spiral disk were then Second, we can compare our results with independent defined as the difference between total fluxes and central measurements from the same ISOCAM raster observa- region fluxes measured in this way. tions (differing in the applied data reduction and pho- tometry), for some galaxies of the sample analyzed by other individuals. In the Cambarre and Camspir samples, 3.5. Errors and photometric consistency there is only one such galaxy, M 83, bright and very ex- Photometric errors cannot be computed rigorously, due tended, for which Vogler et al. (2001) give F15 =20.2Jy to the uncontrolled memory effects of the camera. The and F7 =19.3 Jy, that is to say respectively 1.005 and quoted uncertainties are therefore only indicative. They 1.04 times the values obtained by us: the difference is well are the sum of three components: within the error bars. The fluxes of Virgo galaxies from Boselli et al. (1998) have not yet been published, so that – the readout and photon noises added quadratically, we cannot check their consistency with ours. computed from the readout history of each pixel; We have also processed the 7 and 15 µmmapsofsev- – the error on the background level fitted to the his- eral galaxies from the Sf glx observation program (PI G. togram of off-source pixels (which includes uncertain- Helou), selected from the ISOCAM public archive and ties due to residuals of dark current, flat-field, glitches, added to our sample for the analysis presented in Paper I. remnant images and long-term drifts), also added The maps are not presented here since they have been quadratically; published in Dale et al. (2000) (DSH). The results of our H. Roussel et al.: An atlas of MIR dust emission in spirals 479

Fig. 2. 7 µm fluxes simulated from the spectra (ordinate) ver- sus 7 µm fluxes measured in maps (abscissa). The apertures used are centered on the galactic nuclei and encompass the entire circumnuclear regions (chosen diameters of respectively 24,48,48,84 and 48 for NGC 613, 1097, 1365, 5194 and 5236). Note that the nucleus of NGC 5236 is slightly saturated in the 7 µm map. Error bars include deviation from stabiliza- tion for raster maps and the uncertainty on the zodiacal fore- ground level for spectra. The solid line indicates y = x and the dashed line y =1.2 x own photometry are given in Paper I. Figure 3 shows the confrontation of our fluxes with those of DSH. NGC 6946, which is brighter by one order of magnitude than the other galaxies, is not included, for clarity (DSH obtain fluxes within respectively 8% and 4% of ours at 15 and 7 µm for this object). As seen in Fig. 3, whereas the agree- Fig. 3. Comparison of our photometric results at 15 µm (top) µ ment at 15 µm is reasonable (for all galaxies except three, and 7 m (bottom) with those of Dale et al. (2000) (DSH). Our the difference is less than 15%), the results at 7 µmare error bars include the three components described above but not the systematic flux density calibration uncertainty, which dramatically discrepant, our fluxes being systematically acts equally on both sets of data. The solid line indicates y = x higher than those of DSH. As a result, F15/F7 colors com- and the dotted lines y =(1+α)x,withα varying by steps of 0.1 puted by DSH are always higher than ours. To investigate the origin of this discrepancy, we performed some tests on NGC 986, an extended galaxy with bright circumnu- transient correction. However, none of the changes listed clear regions, for which F15 (DSH) = 1.02 F15 (us) and above is sufficient to account for the large difference in F7 (DSH) = 0.64 F7 (us). the 7 µm fluxes. We also tried to reproduce the photomet- We substituted each step of our data reduction suc- ric method of DSH, but this also can be excluded as the cessively, leaving all the other steps unchanged, according major source of the discrepancy. to what we thought DSH had done from the information As a last possibility, DSH mention that besides using they give on their data processing: use of calibration files calibration data, they also built flat-fields using the ob- for the dark current; use of the deglitcher described by servations themselves, by computing the median of the Starck et al. (1999) with default parameters and without images – but apparently without previously masking the any subsequent masking; use of the “fit3” routine with de- areas where the galaxy falls –; we do not know to which fault parameters for short-term transient correction; sup- galaxies exactly this applies and in which instances they pression of the long-term transient correction; and use of have instead used calibration files. We have thus explored calibration files for the flat-field. this track and created such a flat-field for NGC 986. We We would advise against the empirical “fit3” routine find that this is the only way we can recover the 7 µm flux for photometric purposes (it always finds a solution and given by DSH. artificially produces corrected readout histories which are However, we emphasize that flat-fields should not be flat, but this does not ensure that it converges toward built from the observations without masking the source, the right value). Thus we initially thought that photomet- especially for an extended galaxy with bright central ric discrepancies would be attributable to the short-term regions such as NGC 986, on a small raster: when doing 480 H. Roussel et al.: An atlas of MIR dust emission in spirals so, one strongly overestimates the flat-field response at “D”). Regions in the bar are noted “B” and regions in all the places where bright sources are observed (for the arms, arcs or rings are noted “A”. In some cases, the color 2 × 2 raster of NGC 986 for instance, the nucleus is seen of the central region inside the resolution unit is also given at four places on the detector, and still at three places (“Cr”). Fluxes are expressed in mJy, so that to obtain if the first sky position is removed, as DSH have done). a true ratio of powers emitted inside the 12–18 µmand As a consequence, after dividing by such a “flat-field”, 5–8.5 µm bandpasses (and not a flux density ratio), one the final flux of the galaxy is severely underestimated. If should multiply F15/F7 by ≈0.42. indeed DSH have proceeded this way, this could explain Errors on average disk colors include only photon and why their fluxes are systematically lower than ours. If the readout noises and the uncertainty on the background same processing holds for 15 µm maps, we speculate that level; for all other color measurements, they additionally the discrepancy with our 15 µm fluxes is reduced due to a contain an estimate of the errors due to incomplete sta- compensation effect by the background: at 15 µm, the zo- bilization. We again warn the reader that the latter error diacal emission is much higher than at 7 µm with respect component cannot be derived rigorously: what follows is to the galaxy flux (by a factor 5–6). Since the contrast an attempt to provide an order-of-magnitude estimate. between the background and the galaxy is much lower, Since transients are of a systematic nature (the response the quality of the flat-field response is less affected than of the camera to a given input is perfectly reproducible) at 7 µm. and tend to behave coherently in a well defined region As a further check, one of us (M. Sauvage) inde- (a brightness peak or minimum), we treated them in the pendently reduced the maps and measured the fluxes following way: λ of NGC 986 by a method different to that explained in –Wecallmi the estimated uncertainty due to memory Sect. 3.3. At 7 µm, allowing the isophotal detection limit effects in the pixel i of the final map at the wavelength to vary between 0 and 3 times the background dispersion λ, derived as explained in the begining of Sect. 3.5. Rλ is above the background level, the resulting flux is within the ensemble of pixels belonging to the region of interest. −6% and +8% of our tabulated flux, and the preferred Then the stabilization error on the true total flux F λ of measurement in view of the extension of the galaxy is at that region is
−4%. At 15 µm, these numbers are respectively −14% and λ λ +13%, and −7%. Hence, our various estimates are in good M = mi . λ agreement with each other. i∈R – We consider two cases: either both resulting errors at 7 and 15 µm are treated positively, i.e. fluxes are overesti- 4. Maps λ λ λ λ mated (we measure Fmes ≈ F + M instead of F ), or For each galaxy are shown, from top to bottom: an optical both are treated negatively, i.e. fluxes are underestimated λ λ λ map from the Digitized Sky Survey (with a better contrast (Fmes ≈ F − M ). We assume the same trend in both inset from the second generation survey, if available and if bands because memory effects are systematic in nature, the large-scale image is saturated); the 7 µmmap,towhich and we consider it equally probable that fluxes are overes- we applied a transfer function In,where0.4 ≤ n ≤ 1, so timated or underestimated in the case of an upward flux as to make both faint and bright structures visible with step as well as in the case of a downward step, because the grey scale; F15/F7 colors of a few regions, selected for the detector response can oscillate as a function of time; being regular and bright at 7 or 15 µm,whichareshownas – An asymmetric error bar on the color is then directly c c an illustration of the constancy of colors in disks, except derived. Let us call M+ and M− the bounds of this error  +Mc in a few resolved star formation complexes and in circum- 15 7 + bar: the measured color is noted Fmes/Fmes −Mc .Iftrue nuclear regions. For these color measurements, 7 µmmaps − fluxes are overestimated, were first convolved to the 15 µm angular resolution. As ≈ 15 15 15 15 the astrometry is not more precise than 10 (the ab- Fmes F × 1+M /F 7 = 7 7 7 solute accuracy of ISO, 1–2 , is degraded by ISOCAM’s Fmes F 1+M /F lens wheel jitter), the displacement between both maps 15   F 15 15 7 7 was measured on central regions, by fitting them with a ≈ × 1+M /F − M /F F 7 Gaussian when possible. For some Virgo galaxies not ob- c using a limited development. If Dp is the deviation from served at 7 and 15 µm in concatenated mode but during 15 7 15 7 the true color (Fmes/Fmes − F /F ) induced by an over- different revolutions, there is a rotation between the two c maps of up to 5◦, which was estimated by fitting the few estimation of fluxes and Dn the deviation resulting from an underestimation of fluxes, one obtains usable regular complexes with Gaussians. Apertures were   fixed by the angular resolution: we chose 12 for observa- 15 − 15 15 7 c Fmes M × M − M Dp = 7 7 15 15 7 7 tions with a 3 pixel size, and 18 for a 6 pixel size, i.e. Fmes − M Fmes − M Fmes − M ≈1.25 times the half-power diameter of the point spread and similarly function. We first give the colors of the circumnuclear re-   gion inside the radius defined as explained in Sect. 3.4 15 15 15 7 c Fmes + M × − M M · Dn = 7 7 15 15 + 7 7 (marked “C”), and of the whole averaged disk (marked Fmes + M Fmes + M Fmes + M H. Roussel et al.: An atlas of MIR dust emission in spirals 481

– There are four possibilities: HI-deficient galaxies: they are mostly classified as early c c c c c c If Dp > 0andDn < 0thenM− = Dp and M+ = −Dn. type, due to the abnormal aspect of their spiral arms, and c c c c c c If Dp < 0andDn > 0thenM+ = −Dp and M− = Dn. since the bulge-to-disk ratio criterion plays a minor role c c c c c If Dp > 0andDn > 0thenM− =max[Dp,Dn]and in the classification compared to the resolution of spiral c M+ =0. arms into star formation complexes. c c c c c If Dp < 0andDn < 0thenM+ = −min[Dp,Dn]and Mc =0. − 5.1. Well resolved galaxies Maps are organized as follows: – Strongly barred spirals: Figure 5: strongly barred galaxies arranged in order of NGC 5383 (SBb), 7552 (SBab), 1433 (SBab), 1530 (SBb), decreasing Dbar/D25, the ratio of the deprojected bar 613 (SBbc), 1672 (SBb), 1097 (SBb) and 1365 (SBb). diameter to the disk diameter; The circumnuclear region is extremely bright in the Figure 6: galaxies with weak or no bar arranged in the mid-infrared – always brighter than any complex in the same order; bar or the arms. This is particularly obvious in NGC 7552, Figure 7: the peculiar galaxies; whose bar is relatively faint and where only a hint of the Figure 8: all galaxies of the Virgo cluster sample whatever eastern arm is present, whereas emission from the large the bar strength which are not HI-deficient; central region is very intense. All galaxies of this category Figure 9: HI-deficient Virgo galaxies. are known to possess a nuclear spiral or ring, and some- Maps are oriented with north to the top and east to the times additionally a nuclear bar. All except NGC 1433 left. have a hot-spot nucleus, i.e. giant star formation com- plexes within the central structure. We give here a brief 5. Morphological properties summary of nuclear morphologies: – NGC 5383: nuclear spiral (Buta & Crocker 1993) with In the following, we grouped galaxies according to their hot spots (Sersic 1973); most striking property. The well-resolved regular spirals – NGC 7552: nuclear ring (Feinstein et al. 1990) with hot of the first subsample were arranged in order of decreasing spots – although the nucleus is classified amorphous by Dbar/D25, the ratio of the deprojected bar diameter to the −2 Sersic & Pastoriza (1965), bright complexes are clearly vis- disk major diameter at the blue isophote 25 mag arcsec . ible in the radio continuum maps of Forbes et al. (1994) –; Virgo galaxies are resolved with less detail, are more Dottori & Pastoriza (1986) infer from a population syn- frequently highly inclined and intrinsically smaller and thesis of stellar absorption lines a cycle of three or four fainter, not allowing a meaningful measure of Dbar/D25. successive starbursts; They are separated into two groups differentiated by in- – NGC 1433: nuclear ring and nuclear bar (Buta 1986), teractions with the cluster environment: galaxies with a amorphous nucleus (Sersic & Pastoriza 1965); normal HI content and HI-deficient galaxies. – NGC 1530: nuclear spiral (Buta & Crocker 1993) with Beyond the complexity and distinctive features of all hot spots (Sersic 1973); galaxies shown here, we choose to emphasize in this sec- – NGC 613: nuclear spiral and nuclear bar (Jungwiert tion the generic properties found within each group and do et al. 1997), hot spots (Sersic 1973); not describe in depth each individual object. The morpho- – NGC 1672: nuclear ring with hot spots (Storchi- logical types from the RC3 (de Vaucouleurs et al. 1991) Bergmann et al. 1996), although Sersic & Pastoriza (1965) are indicated. include it among amorphous nuclei; We determined the size of the infrared emitting disk – NGC 1097: circumnuclear starburst ring of diameter (at 7 µm because the sensitivity is better than at 15 µm), ≈ −2 20 1.5 kpc with hot spots, nuclear bar (Friedli et al. defined as the diameter of the isophote 5 µJy arcsec . 1996). The ring is well resolved in the mid-infrared; This is the deepest level that can be reasonably reached – NGC 1365: nuclear spiral and nuclear bar (Jungwiert for all galaxies of the sample, corresponding to between et al. 1997), hot spots (Morgan 1958). one and five times the background 1σ uncertainty (except for NGC 5194 which has a higher background noise). This The bar is detected in all galaxies but NGC 1433, which, level is very similar to the sensitivity reached by Rice et al. besides its central region, displays only very faint and (1988) in 12 µm IRAS maps. The measure was performed patchy emission; in NGC 1365, it is detected but extremely on azimutally averaged surface brightness profiles, taking diffuse. into account the orientation and inclination of the galaxy. We also note that the mid-infrared emission of the bar We cannot see any difference in the mid-infrared size to generally consists of thin bands (as opposed to the stellar optical size ratio between barred and non-barred galaxies, bar which is more oval), shifted towards the leading edge but this ratio tends to increase from early-type to late- (if one assumes that spiral arms are trailing), in agree- type galaxies. More fundamental systematics exist as a ment with the response of other gas tracers to the bar function of gas content, which will be described in Sect. 6. potential. However, these bands do not coincide strictly Indeed, the likely reason for the observed dependence on with dust lanes seen in optical images: dust which is con- Hubble type is the existence of a classification bias for spicuous in optical starlight absorption is not the same as 482 H. Roussel et al.: An atlas of MIR dust emission in spirals dust seen in mid-infrared emission. In fact, absorbing dust with one SBbc, NGC 613). This is not a bias in the sam- lanes are signatures of gas density enhancements, at the ple: long bars are found predominantly among early types location of shocks (Athanassoula 1992). As these shocks (Athanassoula & Martinet 1980; Martin 1995). are strongest in the inner parts of the bar, near the central We finally note the presence of an inner plateau or region, absorption dust lanes are also better developed and lens of diffuse infrared emission delineating a sort of inner show higher contrast in the inner bar, independently of the pseudo-ring in all galaxies of this group, again except brightness of the underlying stellar continuum (this is par- NGC 7552 and 1433. In NGC 1530, the plateau is also seen ticularly well seen in NGC 1530 and 1672); conversely, the in diffuse Hα emission and the pseudo-ring materialized infrared emission is systematically brightest in the outer by Hα knots (Regan et al. 1996). In NGC 613, 1672 and parts of the bar and diffuse near the circumnuclear re- 1097, a real lens is seen in the optical and further outlined gion. The same argument excludes significant collisional by secondary tightly wound arms or incomplete arcs, best excitation of dust in shocks: grains are heated by photons, seen in the infrared. including in regions of large-scale shocks. A further sign of the different nature of absorption and – Weakly or non barred spirals: emission dust lanes is the aspect of the bar of NGC 1365: NGC 5236 (SABc), 6946 (SABcd), 4535 (SABc), 6744 whereas absorption dust lanes show high contrast and are (SABbc), 289 (SBbc), 5457 (SABcd), 4736 (SAab), 5194 displaced far from the bar major axis toward the leading (SAbc). edge, the infrared emission is extremely diffuse and rather In weakly barred spirals, the central region is bright in symmetric about the major axis. the mid-infrared, but contrary to strong bar centers, it is Star formation complexes are generally seen in the bar relatively small and can be rivaled in brightness by com- at varying places, between mid-distance from the nucleus plexes in the arms. The centers of NGC 5236 and 6946 are to the end of the bar (see for instance the bright knots in particularly intense. Several of these galaxies also contain NGC 7552 and 1530). Such complexes inside the bar are nuclear rings, but none seems to harbor hot spots compa- also commonly found in Hα observations (Garc´ıa-Barreto rable to those seen in galaxies of the first group. Here is a et al. 1996; Martin & Friedli 1997). Emission is always list of nuclear morphologies: enhanced at the junction between the bar and spiral arms, – NGC 5236: nuclear ring (Buta & Crocker 1993), amor- even in NGC 1433. This has been interpreted as an effect of phous nucleus (Sersic & Pastoriza 1965); orbit crowding, since it is the place where elongated orbits – NGC 6946: nuclear bar (Zaritsky & Lo 1986); subtended by the bar meet near-circular orbits of the disk – NGC 4535: nuclear ring (Buta & Crocker 1993); (Kenney & Lord 1991). This region therefore favours star – NGC 6744, 289 and 5457: no remarkable structure; formation, unlike the inner bar where the high-velocity – NGC 4736: weak nuclear bar (M¨ollenhoff et al. 1995); shocks are likely to prevent it (Tubbs 1982; Athanassoula – NGC 5194: nuclear ring (Buta & Crocker 1993), which 1992; Reynaud & Downes 1998). is seen in the mid-infrared. In some galaxies (NGC 1530, 1365 and to a lesser ex- tent NGC 5383), spiral arms are clearly seen to begin in The bar is detected in all SABs except NGC 6744, which advance of the bar, i.e. to continue beyond the junction is completely devoid of infrared emission between the nu- with the bar towards the leading edge, which is never the cleus and the inner ring, much like NGC 1433. Unlike case for weakly barred galaxies. This could be due to a strong bars, weak bars do not contain infrared knots: decoupling between the bar and spiral waves, i.e. different their emission is rather unstructured. They consist of thin pattern speeds. Spiral arms generally make sharp angles lanes displaced towards the leading edge, just as in strong with the bar (except the eastern arms of NGC 7552, 613 bars, but are more curved and smoothly connected to spi- and 1672), which is also characteristic of strong bars, as ral arms, in agreement with the notations of Prendergast already noted by Prendergast (1983). (1983). The inner parts of arms show in most cases much In NGC 6946, the bar is rather a fat oval distribution star formation, as evidenced by bright emission and knots of stars, inside which a spiral arm can be traced at the both in the optical and in the mid-infrared (exceptions north. The bar that we see in the infrared is shorter and ◦ are NGC 7552, whose western arm is not detected at all rotated by as much as ≈35 to the leading side of the stel- and whose eastern arm is hardly seen, and NGC 1433, lar oval. It does not coincide with the small northern arm whose entire inner ring is seen near the detection limit). but corresponds well to the ≈1 molecular bar-like distri- Nevertheless, there seems to be a maximum radius encom- bution (Ball et al. 1985; Regan & Vogel 1995), confirmed passing sites of reasonable star formation rates, outside of kinematically by Bonnarel et al. (1988), and in which is ◦ which spiral arms abruptly become fainter, both in opti- embedded a nuclear bar rotated by a further angle of ≈15 cal and infrared images. The clearest and most symmetric to the leading side. example is NGC 1365, but this remark is valid for all. The Zones at the junction between the bar and spiral exhaustion of outer arms could be a signature of the coro- arms are much less conspicuous than in strongly barred tation between the spiral wave and the gas, but it can also galaxies (except the SE junction in NGC 289 which is be related to the fact that all the strongly barred galaxies bright). Brightness peaks are found further out along the presented here are of early type (between SBab and SBb, arms (see in particular NGC 4535). Dust emission follows H. Roussel et al.: An atlas of MIR dust emission in spirals 483 remarkably closely the spiral structure at very large dis- connection to the bar (this complex is clearly delineated tances from the center. Note for instance the strong sim- in our mid-infrared images) and the second arm is em- ilarity between optical and infrared images of NGC 4535, bryonic. In some magellanic barred spirals, the bar center where even the slight brightness enhancement at the does not correspond to the kinematic center (but this is northern edge of the disk, likely due to compression by not the case for NGC 4027, according to Pence et al. 1988). the intracluster gas, is detected at 7 and 15 µm. There This morphology is thought to be driven by tidal interac- is no sharp brightness decrease beyond some radius as in tion (Odewahn 1994), or can result from the cooperation strongly barred spirals, except in NGC 289. This galaxy between spiral waves of modes m =1andm = 2 (Tagger appears to be the closest of its category to strongly barred & Athanassoula 1991), the two interpretations not being galaxies (bright knot at the end of the bar, rather sharp exclusive. angle of spiral arms with the bar, slight advance of the arms on the bar, abrupt decrease in surface brightness be- tween an inner disk and an outer disk), despite its rather 5.2. Virgo subsample short bar. However, we note that the outer disk, whose This subsample includes 9 or 10 spirals with normal HI size defines the normalization to the bar size, is extremely content and 14 or 13 HI-deficient spirals (NGC 4293 is at tenuous, unlike in the other spirals of this group. the limit between the two categories). HI-deficient galaxies NGC 4736, although classified SA, has an oval distor- do not necessarily belong to the cluster core, but can be sion revealed by its velocity field (Bosma et al. 1977), found at large projected distances from M 87. For instance, hence a weak bar. Its metallicity radial gradient is typical NGC 4394, 4450, 4498 and 4580, with high HI deficiencies, of SAB galaxies and could perhaps result from the past are located in the cluster periphery, between 1.3 and 2 Mpc mixing effects of a stronger bar now dissolved (Martin & in projected distance from M 87. Belley 1997). Seven Virgo members of the sample have low blue or H-band luminosities (H data from Boselli et al. 1997, – Peculiar spirals: used because they give an indication on the total stel- NGC 1022 (SBa) and 4691 (SB0/a). lar mass), and their sizes are also intrisically small. They NGC 4691 is described as an “irregular galaxy of the are either normal in their HI content (NGC 4351, 4430 M 82 type” by Sandage (1961) and then as amorphous and 4633) and of rather irregular morphology, except (Sandage & Bedke 1994), based on the chaotic, “intricate NGC 4634 seen edge-on, or deficient with no obvious and extensive dust pattern in the central regions” which is distorsion (NGC 4413, 4491, 4498 and 4506). similar to that in some starburst galaxies such as M 82 and Interestingly, the galaxies presented in this paper suggests “galactic fountain activity”. This interpretation which have a central F15/F7 ratio above 2 are all Virgo is supported by the spectroscopic data of Garc´ıa-Barreto spirals (except NGC 1022), with large HI deficiencies. We et al. (1995), whose I-band and Hα images also reveal a pe- computed the HI deficiency according to Guiderdoni & culiar central structure with four knots. The two strongest Rocca (1985) for the galaxies in our sample (the val- Hα knots are resolved in our mid-infrared images, and are ues are given in Paper I). This quantity is defined as surrounded by two concentrations of molecular gas aligned the logarithmic difference in HI mass surface density be- with them, but at nearly twice the distance from the cen- tween a reference field galaxy sample and the galaxy con- ter (Wiklind et al. 1993). In the mid-infrared, we detect sidered, normalized by the dispersion in the field sam- only the central structures and not the bar or the rest of ple. We have adopted as the threshold for HI dearth the disk. Def > 1.2 (Paper I). For all the galaxies with a cen- NGC 1022 is also peculiar and resembles NGC 4691 tral color F15/F7 > 2 except NGC 1022 (namely NG 4569, in many aspects. Both have extremely faint and smooth 4293, 4388 and 4491), 1.2

7. Summary We have presented complete maps of dust emission at 7 µm of 43 galaxies spanning the spiral sequence from types S0/a to Sdm, at an angular resolution less than 10 and a sensitivity of the order of 5 µJy arcsec−2.Wehave also detailed the data reduction, provided the total fluxes at 7 and 15 µm and outlined the generic morphological properties in this sample. Since there exists an abundant literature on many of these galaxies studied individually, the present atlas has concentrated on common features. The detailed description of the sample and interpretation of the data are given together in Paper I. The morphology is very similar in the mid-infrared and in the optical, except in peculiar galaxies where the dust emission is highly concentrated: the 7 and 15 µmemis- sion follows tightly spiral arms, lenses and individual gi- ant complexes. It appears however less extended than the stellar emission and tends to be reduced in smooth and unstructured spiral arms often seen in the outer disks of early-type galaxies and especially of HI-deficient galaxies. This can be related to low densities of both the gas and the young stellar population. Another noticeable differ- ence concerns the aspect of the bar, where the responses of the interstellar tracers and of the stellar component to the gravitational potential are clearly distinct. Finally, central regions stand out in our maps as entities with particular properties, and are studied in detail in Paper I. The major characteristic of mid-infrared colors, i.e. 15 µmto7µm flux ratios, is that they are remarkably uniform throughout disks, except in some circumnuclear regions and a few bright complexes in spiral arms (the best example of the latter can be found in the outer interacting arms of M 101). The maps will be made available to the com- munity, in fits format, in the ISO public archive (http://www.iso.vilspa.esa.es/). They can be used Fig. 4. Histograms of mid-infrared to optical size ratios as a for studies of the interplay between dust and other compo- function of HI-deficiency and morphological type. Def > 1.2 nents of galaxies, and constitute the basis for two following Def < − . corresponds to severely HI-stripped galaxies and 0 3 papers. to gas-rich galaxies H. Roussel et al.: An atlas of MIR dust emission in spirals 485

40 o 38 o

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Fig. 5. Strongly barred galaxies in order of decreasing bar strength. a) NGC 5383 and 7552, with Dbar/D25 ≈ 0.65 and 0.67. In all figures are displayed, from top to bottom: an optical image from the DSS with, in a corner, a segment showing the orientation of the line of nodes and the inclination value; the 7 µmmap;F15/F7 colors of selected regions (“C”: circumnuclear region; “D”: averaged disk; “Cr”: central resolution element). Locations in the bar (“B”) or the arms (“A”) are indicated on the 7 µmmap, which is scaled identically to the optical map except when a dashed segment outlines the field of view shown below 486 H. Roussel et al.: An atlas of MIR dust emission in spirals

33 o 45 o

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Fig. 5. b) NGC 1433 and 1530, with Dbar/D25 ≈ 0.54 and 0.48 H. Roussel et al.: An atlas of MIR dust emission in spirals 487

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Fig. 5. c) NGC 613 and 1672, with Dbar/D25 ≈ 0.45 and 0.42 488 H. Roussel et al.: An atlas of MIR dust emission in spirals

46 o

40 o

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A3 A7 A4 A1 A6 S A5 A5 B1 B1 B4 B2 A2 B2 B3 A3 A4

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Fig. 5. d) NGC 1097 and 1365, with Dbar/D25 ≈ 0.35 and 0.31. The labels “S” and “N” correspond to the brightest spot in the ring and the nucleus, whose colors were estimated from the images treated with the analog of CLEAN (Sect. 3.4), with no error bars H. Roussel et al.: An atlas of MIR dust emission in spirals 489

24 o 38 o

B1 A5 A4 A1 B1 B2 A4 A1 A2 A3 A5

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Fig. 6. Weakly barred and unbarred galaxies. a) NGC 5236 and 6946, moderately barred with Dbar/D25 ≈ 0.24 and 0.19 490 H. Roussel et al.: An atlas of MIR dust emission in spirals

Fig. 6. a’) Enlargement of the 7 µm map of NGC 6946 H. Roussel et al.: An atlas of MIR dust emission in spirals 491

40 o 50 o

A5 A2

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Fig. 6. b) NGC 4535 and 6744, weakly barred with Dbar/D25 ≈ 0.18 and 0.17 492 H. Roussel et al.: An atlas of MIR dust emission in spirals

o 45 o 18

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A6 A1 A2 A1 A3 A7 A3 B1 A4 A11 A5 A10

Fig. 6. c) NGC 289 and 5457, with Dbar/D25 ≈ 0.15 and 0.06 H. Roussel et al.: An atlas of MIR dust emission in spirals 493

Fig. 6. c’) Enlargement of the 7 µm map of NGC 5457 494 H. Roussel et al.: An atlas of MIR dust emission in spirals

40 o

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A12 A3 Cp A4

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Fig. 6. d) NGC 4736 and 5194, unbarred galaxies (NGC 4736 possesses in fact an oval distorsion). The labels “N” and “Cp” refer to the nucleus of NGC 5194 and the nucleus of its companion, NGC 5195. Note that in NGC 5194, the thin NE and SW outermost parts of both arms seen in the mid-infrared coincide with dust lanes seen in absorption H. Roussel et al.: An atlas of MIR dust emission in spirals 495

Fig. 6. d) Enlargement of the 7 µm map of NGC 5194 496 H. Roussel et al.: An atlas of MIR dust emission in spirals

35 o ~ 0 o

S2 S1

Fig. 7. Peculiar galaxies. a) NGC 1022 and 4691, amorphous barred spirals (possibly merger results). They have Dbar/D25 ≈ 0.32 and 0.43. The labels “S1” and “S2” refer to the two resolved spots in the center of NGC 4691, whose colors were estimated from the images treated with the analog of CLEAN, with no error bars H. Roussel et al.: An atlas of MIR dust emission in spirals 497

40 o

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Acknowledgements. The DSS images shown here were re- Guiderdoni, B., & Rocca-Volmerange, B. 1985, A&A, 151, 108 trieved from the ESO/ST-ECF Archive and were produced at Hameed, S., & Devereux, N. 1999, AJ, 118, 730 the STScI. They are based on photographic data obtained us- H¨ogbom, J. A. 1974, A&AS, 15, 417 ing the Oschin Schmidt Telescope operated by Caltech and the Jungwiert, B., Combes, F., & Axon, D. J. 1997, A&AS, 125, Palomar Observatory, and the UK Schmidt Telescope operated 479 by the Edinburgh Royal Observatory. The ISOCAM data pre- Kenney, J. D. P., & Lord, S. D. 1991, ApJ, 381, 118 sented in this paper were analyzed using and adapting the CIA Malhotra, S., Helou, G., van Buren, D., et al. 1996, A&A, 315, package, a joint development by the ESA Astrophysics Division L161 and the ISOCAM Consortium (led by the PI C. Cesarsky, Martin, P., & Friedli, D. 1997, A&A, 326, 449 Direction des Sciences de la Mati`ere, C.E.A., France). Martin, P., & Belley, J. 1997, A&A, 321, 363 Martin, P. 1995, AJ, 109, 2428 Miville-Deschˆenes, M. A., Boulanger, F., Abergel, A., & References Bernard, J. P. 2000, in ISO beyond point sources: Studies of Athanassoula, E. 1992, MNRAS, 259, 345 extended infrared emission, ESA-SP 455, 27 Athanassoula, E., & Martinet, L. 1980, A&A, 87, L10 M¨ollenhoff, C., Matthias, M., & Gerhard, O. E. 1995, A&A, Ball, R., Sargent, A. I., Scoville, N. Z., Lo, K. Y., & Scott, S. 301, 359 L. 1985, ApJ, 298, L21 Morgan, W. W. 1958, PASP, 70, 364 Bonnarel, F., Boulesteix, J., Georgelin, Y. P., et al. 1988, A&A, Odewahn, S. C. 1994, AJ, 107, 1320 189, 59 Pence, W. D., Taylor, K., Freeman, K. C., de Vaucouleurs, G., Boselli, A., Lequeux, J., Sauvage, M., et al. 1998, A&A, 335, & Atherton, P. 1988, ApJ, 326, 564 53 Prendergast, K. H. 1983, in Internal kinematics and Boselli, A., Tuffs, R. J., Gavazzi, G., Hippelein, H., & Pierini, dynamics of galaxies, IAU Symposium, 215 D. 1997, A&AS, 121, 507 Regan, M. W., Teuben, P. J., & Vogel, S. N. 1996, AJ, 112, Bosma, A., van der Hulst, J. M., & Sullivan, W. T. 1977, A&A, 2549 57, 373 Regan, M. W., & Vogel, S. N. 1995, ApJ, 452, L21 Buta, R., & Crocker, D. A. 1993, AJ, 105, 1344 Reynaud, D., & Downes, D. 1998, A&A, 337, 671 Buta, R. 1986, ApJS, 61, 631 Rice, W., Lonsdale, C. J., Soifer, B. T., et al. 1988, ApJS, 68, Cesarsky, C. J., Abergel, A., Agnese, P., et al. 1996, A&A, 315, 91 L32 Roussel, H., Vigroux, L., Sauvage, M., et al. 2001a, submitted Combes, F., Dupraz, C., Casoli, F., & Pagani, L. 1988, A&A, to A&A, Paper I 203, L9 Roussel, H., Sauvage, M., & Vigroux, L. 2001b, submitted to Coulais, A., & Abergel, A. 2000, A&AS, 141, 533 A&A, Paper II Dale, D. A., Silbermann, N. A., Helou, G., et al. 2000, AJ, 120, Sandage, A., & Bedke, J. 1994, The Carnegie atlas of galaxies 583 Sandage, A. 1961, The Hubble atlas of galaxies D´esert, F.-X., Boulanger, F., & Puget, J. L. 1990, A&A, 237, Sauvage, M., Blommaert, J., Boulanger, F., et al. 1996, A&A, 215 315, L89 Dottori, H. A., & Pastoriza, M. G. 1986, RMxAA, 12, 119 S´ersic, J. L. 1973, PASP, 85, 103 Feinstein, C., Vega, I., Mendez, M., & Forte, J. C. 1990, A&A, S´ersic, J. L., & Pastoriza, M. 1965, PASP, 77, 287 239, 90 Starck, J. L., Abergel, A., Aussel, H., et al. 1999, A&AS, 134, Forbes, D. A., Norris, R. P., Williger, G. M., & Smith, R. C. 135 1994, AJ, 107, 984 Storchi-Bergmann, T., Wilson, A. S., & Baldwin, J. A. 1996, F¨orster-Scheiber, N. M., Laurent, O., Sauvage, M., et al. 2001, ApJ, 460, 252 submitted to A&A Tagger, M., & Athanassoula, E. 1991, IAU Symp., 146, 105 Friedli, D., Wozniak, H., Rieke, M., Martinet, L., & Bratschi, Tubbs, A. D. 1982, ApJ, 255, 458 P. 1996, A&AS, 118, 461 de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H. G., et al. Garc´ıa-Barreto, J. A., Franco, J., Carrillo, R., Venegas, S., & 1991, Third Reference Cat. of Bright Galaxies (RC3) Escalante-Ram´ırez, B. 1996, RMxAA, 32, 89 Vogler, A., Madden, S., Sauvage, M., et al. 2001, in Garc´ıa-Barreto, J. A., Franco, J., Guichard, J., & Carrillo, R. preparation 1995, ApJ, 451, 156 Wiklind, T., Henkel, C., & Sage, L. J. 1993, A&A, 271, 71 Garc´ıa-Barreto, J. A., Downes, D., Combes, F., et al. 1991, Zaritsky, D., & Lo, K. Y. 1986, ApJ, 303, 66 A&A, 252, 19

IV. Echantillon de galaxies 73

IV.3 Suppl´ement `a l’Atlas publi´e : les galaxies des programmes Sf glx et Irgal

Les cartes des galaxies du programme Sf glx ont ´et´e publi´ees dans Dale et al. (2000). Celles que j’ai analys´ees sont cependant incluses ici pour des raisons de commodit´e.

On notera un artefact fr´equent dans ces cartes, visible comme une ligne de discontinuit´e parall`ele aux bords verticaux du d´etecteur, de part et d’autre de laquelle les brillances ne sont pas correctement raccord´ees. Cela est dˆu au fait que pour deux point´es sur quatre, la colonne morte du d´etecteur tombe `a cet endroit de la galaxie. De plus, l’´echantillonnage choisi aggrave cet artefact : comme le d´etecteur est d´eplac´e`achaquepoint´e d’un nombre non entier de pixels, la r´esolution angulaire finale est mal d´etermin´ee et lors de la projection des images, la colonne morte se retrouve projet´ee partiellement sur deux colonnes de pixels.

La pr´esentation est la mˆeme que dans l’atlas principal, mais les diagrammes de couleur sont omis. 74 IV. Echantillon de galaxies 75

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Figure IV.1: en haut : image optique du DSS, avec un segment indiquant l’orientation de la ligne des nœuds et un chiffre donnant l’inclinaison sur la ligne de vis´ee. en bas : cartea7 ` µm. 76

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Chapitre V Formation d’´etoiles et rayonnement de la poussi`ere

Dans The Hubble atlas of galaxies, Sandage (1961) insiste sur le fait que la pr´esence de poussi`ere dans les galaxies (vue en absorption dans les images optiques) est intimement li´ee `ala pr´esence de superg´eantes bleues, ´etoiles jeunes de types O et B. Il en d´eduit que les ´etoiles sont form´ees `a partir de poussi`ere, mais nous dirions plutˆot actuellement, d’une part, que la poussi`ere est ´etroitement associ´ee au gaz `a partir duquel les ´etoiles se constituent, et d’autre part, que la poussi`ere absorbe pr´ef´erentiellement le rayonnement ´energ´etique des ´etoiles jeunes et par cons´equent manifeste sa pr´esence plus clairement lorsque des ´etoiles jeunes sont pr´esentes. La variation du contenu en poussi`ere des galaxies avec le type de populations stellaires dominantes, ou encore avec le type morphologique, n’est pas claire. En particulier, ce n’est que r´ecemment que de grandes quantit´es de poussi`ere froide ont ´et´ed´etect´ees dans les galaxies ext´erieures, par des observations en infrarouge au-del`a de 100 µm (par exemple Alton et al. 1998a).

Le th`eme de l’utilisation de l’´emission infrarouge en tant qu’indicateur quantitatif des taux de formation d’´etoiles a ´et´ebeaucoupd´ebattu sur la base des observations d’IRAS. L’´emission infrarouge fournit une observable commode, parce qu’elle permet de s’affranchir dans une large mesure des effets d’extinction qui sont tr`es complexes. Les estimateurs d’extinction reposant sur la comparaison d’un rapport de raies observ´eaveclerapportintrins`eque d´eduit d’un mod`ele de photoionisation, par exemple, qui sont les plus couramment employ´es, sont biais´es si les deux raies utilis´ees sont toutes deux affect´ees par une extinction substantielle, simplement parce que les r´egions les plus ´eteintes contribuent de fa¸con n´egligeable aux flux, qui sont domin´es par les r´egions d’avant-plan les moins ´eteintes.

La qualit´e du traceur indirect de la formation d’´etoiles qu’est l’´emission de la poussi`ere d´epend `alafoisdesaquantit´e par rapport au contenu stellaire et de la contribution des ´etoiles ´evolu´ees `a son chauffage. Dans ce chapitre, nous allons essayer de tester la validit´e de ce traceur indirect, pour les galaxies spirales, et dans diff´erents r´egimes d’excitation de la poussi`ere.

V.1 Morphologie compar´ee en infrarouge moyen et en Hα

Des cartes dans la raie Hα (incluant le plus souvent les raies [NII]) ont ´et´e obtenues par com- munications priv´ees (des personnes mentionn´ees ci-apr`es) pour quelques galaxies de l’´echantillon parmi les plus ´etendues, et pour lesquelles l’information photom´etrique trouv´ee dans la litt´erature est inexistante ou inad´equate pour notre but. Elles permettent d’une part de compl´eter la base 102 de flux Hα dans des ouvertures directement adapt´ees de celles utilis´ees en infrarouge moyen, et d’autre part de comparer la structure des galaxies dans ces deux traceurs du milieu interstellaire. La r´eponse `a une source ponctuelle (PSF) des observations en Hα montr´ees ici n’´etant pas con- nue, les cartes n’ont pas ´et´econvolu´ees pour les ramener `alar´esolution obtenue en infrarouge. Elles ne sont donc pas directement comparables du point de vue du d´etail de la structure.

Voici un r´esum´e des cartes disponibles : – NGC 5194 = M 51 (Fig. V.1) : carte (Hα +[NII]), extraite de l’´edition ´electronique (ApJ) de Greenawalt et al. (1998), calibr´ee par le flux total donn´e par Young et al. (1996). – NGC 5236 = M 83 (Fig. V.2) : carte en Hα pur fournie par Stuart Ryder, par l’interm´ediaire d’Andreas Vogler, d´ej`a calibr´ee. – NGC 5457 = M 101 (Fig. V.3) : carte (Hα +[NII]) de Fran¸cois Viallefond. Un gradient visible dans le fond a ´et´e approxim´e par un plan αx + βy.Lacartea´et´e calibr´ee par le flux total donn´e par Kennicutt et al. (1994). – NGC 1097 (Fig. V.4) : carte (Hα +[NII]) fournie par Thaisa Storchi-Bergmann, calibr´ee par une mesure d’Osmer et al. (1974) dans une ouverture de 40. – NGC 1365 (Fig. V.5) : carte (Hα +[NII]) fournie par Magnus Naslund, calibr´ee par une mesure d’Osmer et al. (1974) dans une ouverture de 40. – NGC 1530 (Fig. V.6) : carte en Hα pur fournie par Mike Regan, par l’interm´ediaire de Denis Reynaud, d´ej`a calibr´ee. – NGC 1672 (Fig. V.7) : carte (Hα +[NII]) fournie par Thaisa Storchi-Bergmann. Un gradient visible dans le fond a ´et´e approxim´e par un plan αx + βy. La calibration en flux repose sur une mesure d’Osmer et al. (1974) dans une ouverture de 20. – NGC 7552 (Fig. V.8) : cartes (Hα +[NII] + continuum) et continuum fournies par Carlos Feinstein. Pour soustraire le continuum (qui a ´et´eobserv´e dans un filtre centr´e`a 6520 A),˚ le facteur de normalisation a ´et´eestim´e`a l’aide de cinq ´etoiles du champ non satur´ees et en tenant compte des niveaux de fond dans les deux cartes. La calibration en flux ´etait d´ej`afaite. – NGC 1022 (Fig. V.9) : carte (Hα +[NII]) fournie par Antonio Garcia-Barreto (ainsi qu’une carte en bande I). Le continuum soustrait, clairement surestim´e, a ´et´e corrig´e`a l’aide de la carte en bande I. La carte a ´et´e calibr´ee par le flux total donn´e par Hameed & Devereux (1999). – NGC 4691 (Fig. V.10) : carte (Hα +[NII]) fournie par Antonio Garcia-Barreto (ainsi qu’une carte en bande I), calibr´ee au moyen des observations en spectroscopie `a fente publi´ees dans Garc´ıa-Barreto et al. (1995).

Pour M 51, une deuxi`eme carte Hα, provenant de G. Joncas, est disponible. Cependant, toute l’´emission des interbras ainsi que l’´emission diffuse a ´et´etronqu´ee. Par une comparaison avec les flux mesur´es dans la carte de Greenawalt et al. (1998), qui est compl`ete, on peut estimer que l’´emission interbras et diffuse repr´esente 47% de l’´emission totale. Ce chiffre est confirm´e par Thilker et al. (2000), qui ont utilis´e une m´ethode sophistiqu´ee pour soustraire les r´egions HII. J’avais auparavant d´ecompos´elacarte`a7µm de M 51 de mani`ere `as´eparer les bras du reste du disque (selon un seuil en brillance, apr`es avoir “mis `a plat” le profil radial du disque ajust´e par une exponentielle). Il est int´eressant de noter la co¨ıncidence de la proportion de l’´emission interbras et diffuse aux deux longueurs d’onde : j’avais en effet estim´equelesbraset le plateau central de M 51 contribuent pour environ 50%al’´ ` emission totalea7 ` µm. Bien que cette sorte de d´ecomposition soit subjective, les bras de M 51 sont suffisamment fins, brillants et bien d´efinis, pouvant ˆetre suivis de fa¸con ininterrompue sur environ 450◦ de r´evolution, pour V. Formation d’´etoiles et rayonnement de la poussi`ere 103 que la comparaison ci-dessus soit significative.

Les cartes montr´ees ci-apr`es sont clairement d´epourvues de bulbe (au sens o`uilestd´efini dans l’´emission stellaire, comme une composante centrale faiblement aplatie, lisse et se fondant graduellement dans le disque)alafoisenH ` α et en infrarouge moyen (la morphologiea15 ` µm est presque identiquea ` celle vuea7 ` µm), ainsi que not´e pour l’infrarouge par Dale et al. (2000). Elles peuvent ˆetre compar´ees aux images optiques du Digitized Sky Survey qui figurent dans l’Atlas, et, pour les galaxies particuli`eres NGC 1022 et NGC 4691, aux images en bande I de J.A. Garc´ıa-Barreto. L’aspect des bras spiraux, o`u se trouvent la plupart des r´egions de forma- tion d’´etoiles, est tr`es similaire aux deux longueurs d’onde. La structure des r´egions centrales, lorsqu’elle est r´esolue en infrarouge moyen, est ´egalement semblable ; les meilleurs exemples sont l’anneau circumnucl´eairea ` sursaut de formation d’´etoiles de NGC 1097, et la forme asym´etrique du centre de NGC 1365, allong´educˆot´e des bandes de poussi`ere de la barre. Cependant, comme not´e ci-dessus pour M 51, l’´emission n’est pas confin´ee aux bras ou aux r´egions centrales : une fraction substantielle du rayonnement provient de zones diffuses (r´egions interbras, zones in- ternes des barres fortes, structures semblables `a des lentilles dans les galaxies fortement barr´ees de types pr´ecoces), visibles aussi bien en infrarouge qu’en Hα. 104 V. Formation d’´etoiles et rayonnement de la poussi`ere 105

Figure V.1:M51(galaxiedetypeSAbc)danslaraieHα (en haut, carte extraite de Greenawalt et al. 1998) eta7 ` µm (en bas). Le compagnon de M 51 au nord, NGC 5195, a ´et´emasqu´e dans la carte Hα et son intensit´ea´et´etronqu´ee dans la cartea7 ` µm pour rendre la structure de M 51 plus ais´ement visible. 106

Figure V.2: M 83 (galaxie de type SABc) dans la raie Hα (en haut, carte fournie par S. Ryder par l’interm´ediaire d’A. Vogler) eta7 ` µm (en bas). V. Formation d’´etoiles et rayonnement de la poussi`ere 107

Figure V.3: M 101 (galaxie de type SABcd avec une tr`es petite barre) dans la raie Hα (en haut, carte de F. Viallefond) eta7 ` µm (en bas). 108

Figure V.4: NGC 1097 (galaxie de type SBb) dans la raie Hα (en haut, carte fournie par T. Storchi-Bergmann) eta7 ` µm (en bas). V. Formation d’´etoiles et rayonnement de la poussi`ere 109

Figure V.5: NGC 1365 (galaxie de type SBb) dans la raie Hα (en haut, carte fournie par M. Naslund) eta7 ` µm (en bas). 110

Figure V.6: NGC 1530 (galaxie de type SBb) dans la raie Hα (en haut, carte fournie par M. Regan par l’interm´ediaire de D. Reynaud) eta7 ` µm (en bas). V. Formation d’´etoiles et rayonnement de la poussi`ere 111

Figure V.7: NGC 1672 (galaxie de type SBb) dans la raie Hα (en haut, carte fournie par T. Storchi-Bergmann) eta7 ` µm (en bas). 112

Figure V.8: NGC 7552 (galaxie de type SBab) dans la raie Hα (en haut, carte fournie par C. Feinstein) eta7 ` µm (en bas). V. Formation d’´etoiles et rayonnement de la poussi`ere 113

Figure V.9: NGC 1022 (galaxie amorphe de type SBa) dans la bande I (en haut) et dans la raie Hα (au milieu, cartes fournies par J.A. Garc´ıa-Barreto) et `a7µm (en bas). 114

Figure V.10: NGC 4691 (galaxie amorphe de type SB0/a) dans la bande I (en haut) et dans la raie Hα (au milieu, cartes fournies par J.A. Garc´ıa-Barreto) et `a7µm (en bas). V. Formation d’´etoiles et rayonnement de la poussi`ere 115

V.2 Dans les disques

Nous verrons au chapitre VI que la distribution spectrale d’´energie dans l’infrarouge moyen change radicalement des disques (consid´er´es comme une composante unique) aux r´egions cen- trales, o`u le continuum des tr`es petits grains est fr´equemment d´ecal´e vers les courtes longueurs d’onde, r´ev´elant une distribution en temp´erature plus ´etendue vers des valeurs hautes. Cela fait que dans les r´egions centrales, nos observations m´elangent plusieurs phases de poussi`ere, alors que dans les disques, nous voyons principalement une seule phase, les porteurs des bandes aromatiques. Cette simplification du probl`eme est une raison pour se restreindre aux disques dans un premier temps.

De plus, tous les traceurs de formation d’´etoiles plus directs que l’´emission infrarouge ou radio thermique sont sujetsa ` une extinction importante (c’est le cas des raies de recombinaison de Balmer) ou sont relativement difficiles `a observer parce qu’ils requi`erent une grande sensibilit´e dans l’infrarouge proche (par exemple les raies des s´eries de Paschen et Brackett). Or les densit´es de surface et les profondeurs optiques sont g´en´eralement beaucoup plus ´elev´ees dans les r´egions circumnucl´eaires. Par exemple, Bianchi et al. (2000a), en utilisant un mod`ele de transfert radiatif appliqu´e`a une distribution spectrale d’´energie compl`ete de l’ultraviolet lointain `a l’infrarouge lointain (de 910 A`˚ a 850 µm) de la galaxie NGC 6946, ont trouv´e une absorption centrale de l’ordre de 5 magnitudes dans le bleu, contre une absorption moyenne AV ≈ 0.5 mag. Si l’on mod´elise la distribution de la poussi`ere par rapport aux ´etoiles non plus comme homog`ene mais structur´ee en nuages, l’absorption diminue au plus de 1 mag (Bianchi et al. 2000b).

Enfin, les m´ecanismes et efficacit´es de formation d’´etoiles sont tr`es dissemblables dans les disques et dans les r´egions centrales. La premi`ere diff´erence est li´ee aux densit´es de gaz, qui sont plus grandes par 1a ` 3 ordres de magnitude dans les centres (Kennicutt 1998b). Le plus souvent, l’essentiel du gaz circumnucl´eaire est observ´e sous forme mol´eculaire, et la distribution du gaz atomique neutre est parfois d´eprim´ee dans cette r´egion centrale. Tacconi & Young (1986), par exemple, ont mis en ´evidence ce genre de s´egr´egation des phases du gaz interstellaire dans la galaxie (riche en gaz atomique) NGC 6946. Wang (1990),a ` partir d’une compilation de donn´ees HI et CO de 15 galaxies, confirme que le gaz mol´eculaire domine presque toujours la densit´e de gaz au centre, except´e dans les galaxies de faible masse qui contiennent plus de gaz sous forme atomique. La deuxi`eme diff´erence essentielle concerne la vitesse angulaire de rotation, qui d´ecroˆıt du centre vers l’ext´erieur. L’´echelle de temps dynamique (la p´eriode de rotation du gaz) est donc beaucoup plus petite dans les r´egions centrales.

Kennicutt (1998b), utilisant un ´echantillon qui regroupe des galaxies spirales normales et des starbursts circumnucl´eaires ou se produisant `al’´echelle d’une galaxie enti`ere, a montr´equela loi de formation d’´etoiles, reliant la densit´e de colonne moyenne du gaz au taux de formation d’´etoiles par unit´e de surface, reste la mˆeme sur plus de trois ordres de magnitudes et peut (1.3−1.5) se param´etrer de fa¸con ´equivalente comme une loi de Schmidt, ΣSFR ∝ Σgaz , ou comme ΣSFR ∝ ΣgazΩgaz,o`uΩgaz est la vitesse angulaire de rotation du gaz. Cela implique que l’efficacit´e de formation d’´etoiles (c’est `a dire le taux de formation d’´etoiles par unit´edemasse de gaz disponible) est d’autant plus grande que la densit´e de gaz l’est aussi. Dans les r´egions centrales des galaxies, l’efficacit´e de formation d’´etoiles est donc beaucoup plus ´elev´ee, et les effets de r´etroaction sur le milieu interstellaire plus importants. 116

Si le gaz est suffisamment dense, il n’est pas besoin de m´ecanisme sp´ecial comme le passage d’une onde de densit´e pour induire une contraction des nuages mol´eculaires, comme dans les bras spiraux. Dans des anneaux associ´es `adesr´esonances internes de Lindblad, existent des orbites chaotiques qui favorisent la collision des nuages.

V.2.1 Publication

The relationship between star formation rates and mid-infrared emission in galactic disks H. Roussel, M. Sauvage & L. Vigroux accept´e pour publication dans Astronomy & Astrophysics (2001)

Je reproduis ici l’abr´eg´e puis les conclusions de cet article :

Les brillances de surface moyennes en Hα et en infrarouge moyen sont compar´ees dans les disques d’un ´echantillon de galaxies spirales proches observ´ees par ISOCAM. Il en r´esulte que, dans les disques spiraux, l’´emission de la poussi`ere `a7et15µm constitue un traceur valable de la formation d’´etoiles. Le fait que le rapport des fluxa15et7 ` µm soit quasiment constant dans des conditions d’excitation globales diverses indique une origine commune, les porteurs des bandes aromatiques infrarouges, et implique qu’`a ces longueurs d’onde, l’´emission de la poussi`ere dans les disques de galaxies normales est domin´ee par les r´egions de photodissociation, et non par les r´egions HII elles-mˆemes.

Cette nouvelle corr´elation entre l’infrarouge moyen et la raie Hα est utilis´ee pour examiner la nature de la relation entre l’infrarouge lointain (60 et 100 µm) et Hα. Bien que les r´egions centrales ne soient pas s´eparables du disque dans l’infrarouge lointain, nous montrons qu’une contribution circumnucl´eaireal’´ ` emission de la poussi`ere, sans r´eel ´equivalent en Hα,estle responsableleplusprobabledelanon-lin´earit´e connue entre les flux en infrarouge lointain et en Hα des galaxies spirales.

Nous d´eduisons d’une calibration primaire en flux Hα de la litt´erature une calibration des fluxa7et15 ` µm en termes de taux de formation d’´etoiles. Nous insistons aussi sur les limites d’applicabilit´e de cette conversion, qui ne devrait pas ˆetre appliqu´ee sans pr´ecautions `ades galaxies de nature inconnue.

Les flux en infrarouge moyen peuvent ˆetre consid´er´es comme des indicateurs de formation d’´etoiles fiables dans des environnements d’activit´emod´er´ee tels que les disques de galaxies spirales. Les spectres entre 5 et 18 µm des disques sont domin´es par des bandes aromatiques, qui sont li´ees beaucoup plus ´etroitement au rayonnement des ´etoiles jeunes qu’`alalumi`ere bleue, qui provient d’´etoiles de toutes masses et de tous ˆages.

La calibration en termes de taux de formation d’´etoiles que nous proposons d´epend bien sˆur de la fonction de masse initiale adopt´ee, mais surtout de la correction d’extinction appliqu´ee aux flux Hα. La dispersion dans les flux Hα par rapportalacorr´ ` elation lin´eaire, 0.19 dex, est bien plus petite que la correction d’extinction de 0.44 dex. Nous trouvons une dispersion bivariationnelle d’un facteur 1.37 autour de la corr´elation lin´eaire, et une dispersion dans chaque variable d’un V. Formation d’´etoiles et rayonnement de la poussi`ere 117 facteur 1.56,a7 ` µm comme `a15µm. Malgr´e cette large dispersion, notre calibration peut ˆetre employ´ee de fa¸con significative pour de grands ´echantillons, car la densit´e de taux de formation d’´etoiles a une dynamique bien plus ´etendue (dans notre ´echantillon, elle varie par plus d’un facteur 50).

N´eanmoins, il faut gardera ` l’esprit plusieurs limitations : –Lapoussi`ere peut ˆetre chauff´ee par le rayonnement provenant de r´egions HII `a grande distance ; elle peut aussi, dans des r´egions diffuses, ˆetre chauff´ee principalement par des populations stel- laires ´evolu´ees. La relation entre les flux en infrarouge moyen et les taux de formation d’´etoiles est donc certainement beaucoup plus complexe dans des r´egions spatiales limit´ees que si l’on consid`ere l’´emission int´egr´ee. – Concernant les galaxies lointaines d´etect´ees dans les relev´es profonds, seuls les flux totaux sont mesurables, et peuvent provenir de fa¸con pr´edominante des r´egions galactiques centrales. Dans ce cas, le lien entre la formation d’´etoiles et l’´emission en infrarouge moyen peut aussi diff´erer radicalement. En effet, l’utilisation des flux totaux, en infrarouge lointain commea15 ` µm, in- troduit une non-lin´earit´e dans la corr´elation avec les flux Hα. Plusieurs effets interviennent sans doute : un ´etat thermodynamique des grains diff´erent de celui dans les disques ; une absorp- tion plus ´elev´ee en Hα ; une contribution dominante de populations post-starburst du bulbe au chauffage de la poussi`ere, en l’absence de formation d’´etoiles cons´equente. – Les galaxies spirales de notre ´echantillon sont probablement de m´etallicit´eprochedelavaleur solaire. Cependant, la composition de la poussi`ere est modifi´ee par une d´eficience en m´etaux (qui est observ´ee dans les galaxies naines irr´eguli`eres et bleues compactes). Si l’abondance des grains carbon´es chute, l’´emission en infrarouge moyen, dans un champ de rayonnement donn´e, sera r´eduite (Sauvage et al. 1990 ; Boselli et al. 1998). Les bandes aromatiques sont souvent absentes ou de faible intensit´e dans les galaxies naines, `alafois`a cause de cet effet et de la destruction des particules par les photons ultraviolets durs, tr`es p´en´etrants dans les milieux sous-m´etalliques (Madden 2000). – Dans des environnements `am´etallicit´e normale mais tr`es actifs, les porteurs des bandes aroma- tiques peuvent aussi ˆetre d´etruits ou subir des transformations chimiques, mais ces effets n’ont pas encore ´et´equantifi´es.

Malgr´e tout, cette relation entre flux en infrarouge moyen et taux de formation d’´etoiles, dont l’applicabilit´en’a´et´ed´emontr´ee que dans les disques normaux, a une utilit´epourinterpr´eter les relev´es de galaxies r´ealis´es dans les filtres LW3 (15 µm) et LW2 (7 µm). A des d´ecalages spectraux de l’ordre de 1.2, l’´emission au repos en LW2 se retrouve dans la bande passante de LW3. Notre calibration doit donc fournir une limite inf´erieure des taux de formation d’´etoiles, car pour des activit´es de formation d’´etoiles sup´erieures `a celles de notre ´echantillon, la redis- tribution d’´energie favorise la bande LW3 (les tr`es petits grains r´e´emettent plus d’´energie). Les observations pr´esent´ees par Boulanger et al. (1998) de r´egions Galactiques (un nuage diffus et quatre r´egions de photodissociation) montrent que l’´emission dans les bandes aromatiques augmentent avec la densit´ed’´energie du rayonnement ultraviolet, lin´eairement aux basses den- sit´es d’´energie puis plus lentement. A cause d’effets de dilution, la densit´ed’´energie `a laquelle se produit cette transition, au-dessus de 103 fois la valeur du voisinage solaire, est incertaine. Cependant, le mˆeme type de comportement doit ˆetre observable dans l’´emission int´egr´ee des galaxies. 118

Un crit`ere de validit´e simple de notre relation, pour des galaxies non r´esolues, serait une couleur au repos F15/F7 de l’ordre de 1, garantissant que les r´egions centrales contribuent faible- ment aux flux totaux, ou abritent des populations stellaires comparablesa ` celles des disques (non starbursts).

Astronomy Astrophysics manuscript no

will b e inserted by hand later

The relationship b etween starformation rates

and midinfrared emission in galactic disks

H Roussel M Sauvage L Vigroux and A Bosma

DAPNIAService dAstrophysique CEASaclay GifsurYvette cedex France

Observatoire de Marseille Place Le Verrier Marseille cedex France

Received February Accepted March

Abstract The H and midinfrared mean disk surface brightnesses are compared in a sample of nearby spirals

observed by ISOCAM This shows that in spiral disks dust emission at and
mprovides a reasonable star

formation tracer The fact that the to
m ux ratio is nearly constantinvarious global exciting conditions

indicates a common origin namely the aromatic infrared band carriers and implies that at these wavelengths

dust emission from the disks of normal galaxies is dominated by photo disso ciati on regions and not byHII regions

themselves

We use this newlyfound correlation b etween the midinfrared and the H line to investigate the nature of the link

between the farinfrared and
m and H Although the separation of the central regions from the disk is

imp ossible to achieve in the farinfrared weshowthatacircumnuclear contribution to the dust emission having

no equivalent counterpart in H is most likely resp onsible for the wellknown nonlineari tybetween farinfrared

uxes in spiral galaxies and H

We derive a calibration of and
m uxes in terms of star formation rates from a primary calibration of H

in the literature and also outline the applicabil ity limits of the prop osed conversion which should not b e blindly

extrap olated to ob jects whose nature is unknown

Key words galaxies spiral galaxies ISM stars formation infrared ISM

Intro duction b oth comp onents are out of thermal equilibrium and un

dergo large temp erature uctuations of several hundred

Whether midinfrared emission can b e considered a re

K

liable tracer of the massive stellar content of normal and

Sturm et al haveprovided a census of the con

isolated spirals is still unclear The accepted interpretation

tinuum emission and of emission features UIBs and ionic

of midIR sp ectra of galaxies see the review by Puget

lines found from to mintypical starburst galax

Leger Desert et al is that they consist primar

ies and which are p otentially present in our data as well

ily of a comp osite of a featureless continuum and of a fam

Emission from the envelop es of cold stars can also con

ily of aromatic bands the socalled unidentied infrared

tribute in the m bandpass but it is negligible in our

bands UIBs The continuum emission is attributed to

sample except p ossibly in the disks of two SaSa galax

very small grains VSGs Desert et al of whichlit

ies

tle is known while various carb onaceous materials have

b een suggested as candidates to carry the UIBs among

This dual nature of the midinfrared emission pro

whichthePAH mo del p olycyclic aromatic hydro carb ons

duced mainly bytwo dust phases UIB carriers and VSGs

of Leger Puget has b een a longtime favorite

makes the existence of a direct link with massive stars un

However the recentwork of Boulanger et al in

likely A further complication is that even if b oth sp ecies

dicates that UIB carriers are likely aggregates of several

are predominantly heated by high energy radiation their

hundred atoms rather than macromolecules It is imp or

excitation by optical and near ultraviolet photons maybe

tant to realize that under most radiation eld conditions

signicantinenvironments where old stellar p opulations

dominate Indeed aromatic bands are ubiquitous in the

Send oprint requests to H Roussel email hrousselceafr

diuse interstellar medium Giard et al Mattila et

Based on observations with ISO an ESA pro ject with in

al and are also observed in regions where the ultra

struments funded by ESA Memb er States esp ecially the PI

violet radiation density is insucient to account for their

countries France Germany the Netherlands and the United

heating Sellgren et al Boulade et al Uchida Kingdom and with the participation of ISAS and NASA

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

et al When observing extragalactic ob jects a and b hereafter Atlas and Pap er I In this pap er

emission arising in star forming regions is mixed with that we use this advantage to restrict ourselves to the study of

arising in the interstellar medium asso ciated with more galactic disks Our motivation for that is manifold

evolved stars Therefore the accuracy with whichmid The midinfrared colors of galaxies in our sample are

infrared emission traces star formation should in principle relatively uniform in the disks while the circumnuclear

dep end on the balance b etween these two heating sources regions can show strong
m emission excesses suggest

This can b e checked either byinvestigating spatially re ing the existence of a dierent thermo dynamical state of

solved galaxies or by building global energy budgets As dust in the central parts of galaxies

we will show later the ux fraction due to heating byold Since wewant to establish a calibration of midinfrared

stellar p opulations is generally quite small in spiral galax uxes in terms of star formation rates weneedtocom

ies as already noted by Lemke et al pare them with a direct primary star formation tracer and

An attempt to link the midinfrared emission of galax so far few of them sample equally well the disk and the

ies with recent star formation is however encouraged by nuclear regions of galaxies mainly b ecause of extinction

the following fact resolved observations of individual re It has b een shown Kennicutt a that star forma

gions in our Galaxy have revealed that aromatic bands are tion pro cesses and physical conditions prevailing in nuclei

closely asso ciated with the photo disso ciation shells and are widely dierent from those of disks

surfaces of molecular clouds in the vicinityofHII regions or

In Sect we present our sample and the metho ds

hot stars while the VSG continuum strongly p eaks inside

used to collect the photometric information needed for our

HII regions Cesarsky et al Tran Verstraete

analysis Sect demonstrates the validity of midinfrared

et al Given that b oth typ es of sites are intimately

uxes as star formation tracers in galactic disks In Sect

linked with the presence of massive stars a strong cou

we discuss the applicability limits of our calibration and

pling b etween midinfrared emission and presentday star

the implications of our ndings on the interpretation of

formation could exist

the farinfrared emission of galaxies

An invaluable advantage of infrared observations over

optical recombination lines or the ultraviolet continuum

resides in their muchlower sensitivitytointerstellar ex

The galaxy sample and photometric data

tinction thus providing insights into obscured star form

ing regions Besides if it is conrmed that young stars are

The sample of spiral galaxies considered here is made from

the ma jor heating source of dust emitting at and m

the partial merging of ve ISOCAM programs

midinfrared uxes could provide a more acceptable star

Camspir PI L Vigroux which mapp ed nearbyvery

formation tracer than farinfrared uxes since the latter

large spiral galaxies extensively observed in other inter

have b een shown to contain a cirrus comp onent Helou

stellar tracers allowing detailed spatial analyses

which dominates the mean emission from morpho

Cambarre PI C Bonoli which mapp ed barred spiral

logical typ es Sa to Sc Sauvage Thuan and is

galaxies selected to span the variety of bar and Hubble

resp onsible for a strong nonlinearity in the correlation b e

typ es and a large range of infrared luminosities

tween farinfrared and H uxes A vast literature covers

resolved spirals from the complete survey of the Virgo

the advantages and limitations of interpreting farinfrared

program PI J Lequeux see Boselli et al

emission as a star formation indicator and includes for

nonSeyfert spirals with no strong signs of tidal inter

instance LonsdalePersson Helou Devereux

action from the Sf glx program PI G Helou see Dale et

Young and Smith Harvey A summary

al which selected galaxies sampling the IRAS color

of the issues in question can b e found eg in Sauvage

color diagram Helou

Thuan or in Kennicutt b

identically chosen spirals from the Galir program PI

On the other hand since survey programs p erformed

T Onaka that aimed at constructing infrared sp ectral

with ISO have fo cussed on observations at and
m in

energy distributions of normal galaxies in preparation for

vestigating the relationship b etween the emission in these

the Japanese mission IRTS

bandpasses and star formation rates would b e very helpful

for their physical interpretation The resulting sample comprises galaxies spanning

To tackle this question we use a sample of galaxies the whole de Vaucouleurs spiral sequence from Sa to

observed by ISOCAM in two broadband lters centered at Sdm All were observed in raster mo de in the two lters

and
m All the galaxies b eing nearbytheachieved LW centered at
m
m and LW centered at

spatial resolution is sucient to delineate distinct struc
m
m with a pixel size of or General

tural entities spiral arms giantHII complexes the cir information and a deep er discussion of the midinfrared

cumnuclear concentration etc Distances to the sample prop erties of these galaxies can b e found in Pap er I to

galaxies range from to Mp c which translates into lin gether with the sp ectra b etween and
m observed in

ear resolutions of p c to kp c at
m full width at ve of them The detailed description of data reduction

half maximum of the p oint spread function This allows a app ears in the Atlas that also presents the
m maps

clear dierentiation of the midinfrared prop erties of cen We simply note here that we pro cessed all maps in a ho

tral regions and disks a study describ ed in Roussel et al mogeneous way including those already published

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

signicantly smaller than the size of the circumnuclear H data

region in the midinfrared this concerns galaxies out

Since the goal of this pap er is to assess the reliabilityof

of wechecked that the circumnuclear F F color

midinfrared emission as a star formation tracer we need

is low b elow ie shares the main characteristic of

to collect data on a primary indicator Recombination

disks This ensures that we are not intro ducing a strong

lines which trace the existence of massive stars are an

bias b ecause the color indicates that the star formation

obvious choice wellestablished calibrations in terms of

pro cess and extinction should b e close to those found in

star formation rate SFR exist and their pro duction by

disks The ap erture used byPogge from whom

p ostAGB stars as seen in ellipticals Binette et al

the central H uxes of galaxies are obtained is not

is negligible in starforming galaxies Kennicutt a

given explicitly but could b e estimated from his H im

We therefore searched the literature for integrated

ages except for VCC and VCC for whichwehave

H photometry In addition some H maps were kindly

assumed that it is equal to the infrared size of the central

made available to us by JA GarcaBarreto for NGC

region This could b e problematic in the case of VCC

and NGC published in GarcaBarreto et al

whose central color is high F F However all

T StorchiBergmann for NGC and NGC pub

these uncertainties remain a negligible source of error with

lished in StorchiBergmann et al M Naslund

resp ect to the extinction correction

for NGC published in Lindblad courtesy of

Most H measurements at A include the two

S Jorsater M Naslund and JJ Hester C Feinstein

neighb oring NII lines the most intense at Aand

for NGC data published in Feinstein MW

another one at A We applied the same correction as

Regan via D Reynaud for NGC published in Regan

Kennicutt for average NII contamination in disk

et al SD Ryder via A Vogler for NGC

HII regions of the total ux The NIIH ratio is in

published in Ryder et al Vogler et al F

general higher in central regions than in disk HII regions

Viallefond for NGC Some of these maps have b een

eg Brand et al but wehave removed central

corrected for a spatial gradient using the background or

NIIH uxes and ratios in disks are little disp ersed

for an oversubtraction of the continuum emission using

Kennicutt Kent

Iband images In some cases we also p erformed the ux

We also made use of the value given by Kennicutt

calibration using data from the literature inside various

for average and uniform extinction in the H line

ap ertures

exp ected to b e the ma jor source of uncertainty since it

Concerning galaxies for whichwehavenomapthe

amounts to mag It is clear that a uniform extinction

bulk of the data comes from Young et al Wenote

correction is in principle very far from the true correc

that this reference provides H NII uxes systemat

tion that should b e applied However we can rst exp ect

ically higher than those of Kennicutt Kent for

that regions where the extinction most signicantly de

the galaxies in common by a factor ranging from ab out

parts from this value are lo cated in the central parts of

one to two Wethus preferred to adopt data from Young

galaxies whichwehave excluded from the present analy

et al or other references but for six of our galax

sis Second it is on H data corrected in this way that SFR

ies they were taken from Kennicutt Kent we

calibrations are built And third apart from observation

have corrected them by a factor following Kennicutt

ally deriving the extinction in each ob ject it is not p ossible

bs prescription and wehavechecked that the op

to dene a correction scheme eg based on Hubble typ e

tical diameter of these galaxies is less than or comparable

or on global Balmer decrement that do es not intro duce

to the H ap erture used except for VCC the optical

as much uncertainty and bias as it supp osedly removes

size and H ap erture are resp ectively and

We therefore cho ose to conne ourselves to this uniform

Since the central regions of most galaxies in our sample correction scheme b earing in mind that our conclusions

stand out in the midinfrared as having dierent prop erties are relative to this metho d of correcting H data in order

from the disk see Pap er I and the Atlas and as the con to estimate the SFR

tamination of H uxes bynuclear regions can b e signi Control on the bias that weintro duce therebycanbe

cant in galaxies harb oring nonstellar activity or starburst found in an examination of the H to m ux ratio

and for all the reasons emphasized in Sect wechose to since we apply a uniform correction on the H data it

exclude the central regions from b oth midinfrared and H makes no dierence here whether these are corrected or

not The variation of the ratio of H to m uxes as measurements For this purp ose we used matched ap er

a function of the inclination estimated from kinematical tures whichwere dictated by the available H data in the

data or if unavailable from the ratio of ma jor to minor literature We aimed at subtracting circumnuclear uxes

isophotal diameters is a pure scatter diagram We also of suciently large a region to match that region inside

checked that separating our sample into two morpholog which most of a p ossible m excess is lo cated It was

ical classes SaSb and Sb cSdm did not result straightforward to achieve this when we could directly p er

in signicant a dierence in the H to m ux ratio form measurements on H maps However this could not

the logarithmic means in the two subsamples are resp ec be achieved in practice for all galaxies due to the diculty

tively
and
Finally no trend of nding suitable H data When suchnuclear data are

canbeseeninaplotoftheH to m ux ratio versus ailable or were measured only inside an ap erture not av

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

tral regions the most obvious one is a scale eect ie the sizenormalized H ux see section for a denition

large or bright galaxies tend to b e bright at all wave of this quantity Since the m emission is much less

lengths and comparing them with small and faint galaxies prone to absorption we are therefore condentthatno

intro duces articial correlation of the data Another bias systematic variation in the extinction aects the H data

results from the use of luminosities instead of uxes an in Furthermore the values of F F H in our sample are

correct estimation of distances intro duces disp ersion and all compatible with mo derate absorption if compared with

the presence of the distance squared on b oth axes also the values observed in M Sauvage et al

pro duces an articial correlation We therefore haveto

Galactic extinction was corrected using the blue ab

normalize our data by another galaxy prop erty indep en

sorptions listed in the RC together with the extinction

dent of b oth midinfrared and H emission Wechose for

curve of Cardelli et al The galaxies for which suit

such a quantity the disk area from the ma jor diameter

able H data were found are listed in Tablewiththeir

at the blue isophote mag arcsec dened in the

total and central uxes

B

RC We stress that even though the normalization of

uxes by the disk area gives quantities that are formally

MidIR photometry

surface brightnesses these should not b e identied with

midinfrared or H surface brightnesses The normaliza

Wehave measured total uxes as explained in the Atlas

tion is only used to avoid the scaleeect in our sample

and central uxes inside the same ap erture as that used

In the following we will refer to the quantities obtained

for H in images treated with an algorithm designed to

in this way as sizenormalized uxes

correct for dilution eects also describ ed in the Atlas

We applied two tting pro cedures The rst one is the

The disk ux is then the dierence b etween the total ux

classical minimization of squared distances to a line with

and the corrected nuclear ux The resulting central ux

an equal treatment of b oth variables The second one is the

fractions whichhave b een removed and disk uxes are

minimization of absolute values instead of squares again

given in Table

bivariate and is more robust to outlier p oints Fig a

As already mentioned the ap erture of H measure

shows the dep endence of the sizenormalized m on the

ments matches reasonably well the size of the circumnu

sizenormalized H uxes The b est least squares t im

clear concentration in the images corrected for dilution

F scales as F H with a correlation co ef plies that

We call the attention of our readers to the fact that the

cient of the interval for the slop e is

size of the ap erture used to measure central midIR uxes

The results for m are quite similar to those for m

in this pap er is dierent from that used in Pap er I In

Fig b with approximately the same p ower law That

Pap er I since we did not have the constrainttomatc h

the and m ux densities app ear interchangeable in

the midIR ap erture to that available for data taken at

Fig may come at rst as a surprise given the already

a dierentwavelength the size of the central regions was

growing amount of literature data indicating that regions

measured on nondeconvolved midIR surface brightness

of high star formation activity app ear as regions of en

proles The ap erture sizes given in Table are thus dif

hanced m emission with resp ect to m Vigroux et

ferent from those used in Pap er I

al Sauvage et al Dale et al However it

Typical errors on disk uxes are exp ected to b e of the

is also clear that not all star forming regions showa

order of or mainly due to the camera memory

m excess see eg the color data in Dale et al and

eects and to ux calibration uncertainties

our Atlas and the excess app ears only ab ove a certain

threshold in starformation activity This last p ointisam

The starformation rate scaling in galactic disks

ply demonstrated by the socalled ISOIRAS colorcolor

diagram Vigroux et al Helou that plots the

In M and NGC for example there exists a strik

F F ISO color versus the F F IRAS color for most

ing corresp ondence b etween the spatial distributions of

of the F F range the F F color is nearly constant

H and m emission Sauvage et al Smith

and around and only starts to increase for the hottest

When we compare in detail our and m maps with the

F F colors corresp onding to starburst and interact

H maps at our disp osal all structures such as rings bars

ing galaxies For normal spiral galaxies such as those in

and arms are very similar at the three wavelengths even

our sample the F F color do es not signicantly devi

without correction for the dierent angular resolutions

ate from In fact weshowinPap er I that changes of

and sensitivities Thus it is tempting to check the robust

the global F F color of spirals are strictly due to the

ness of this correlation in a more quantitative approach

circumnuclear regions which are not included in Fig

If it holds for total uxes it would indicate a strong rela

Spiral disks exhibit a F F color of in ux

tionship b etween midinfrared dust emission and the SFR

density units ie in Jy

We rst have to cancel identiable bias sources in our

The fact that the and m uxes show similar vari

data In addition to the varying contribution of the cen

ations with radiation density in disks is somewhat puz

zling as they were originally thought to b ehave quite dif

The stabilizati on correction that we applied uses the latest

ferently since the main emission sites of aromatic bands

available technique taking into account the detector character

and VSGs resp ectively photo disso ciation regions and HII istics describ ed in Coulais Ab ergel

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

L m L regions are distinct A likely explanation is that when

UIB bol

averaged in disks the m emission is no longer domi

with L W this assumes solar abun

bol nated by VSGsasinHII regions but by a part of the fea

dances and a Salp eter IMF ranging from to M

ture comp osed of the aromatic bands at and m

Flux densities at and mwere converted into lumi

and thus shares a common origin with the memis

nosities using bandpasses of THz and THz re

sion This is what can b e seen in our few midIR sp ectra

sp ectively These formulas are applicable only when the

Pap er I Other fainter UIBs are additionally presentin

midinfrared emission is dominated by UIBs with a negli

the m range see for instance Hony et al A

gible VSG continuum which is the case in disks of galax

further conrmation of the common origin of the and

ies but is not always veried at m in galactic central

m uxes in disks can b e found in the fact that the de

regions for instance

p endence of F F on the IRAS color F F is weak for

In our sample sizenormalized SFRs in disks we

low ratios F F and b egins to strengthen only for

mean here physical regions dened in Pap er I range b e

relatively hot colors as the m ux density is clearly

tween ab out and M kyr kp c fromVCC

due to VSGs this indicates another origin for the m

to NGC and SFRs of disks b etween ab out and

ux density in the low F F range

M yr

Fig thus implies that on the scale of galactic disks in

We emphasize that the relationship found b etween

the midinfrared HII regions are seen only through their

dust emission and the H recombination line is unique

eect of globally increasing the interstellar radiation eld

and is not repro duced if the H emission is replaced byan

Since impulsive heating of aromatic band carriers by single

observable tracing more evolved stellar p opulations such

photons causes the shap e of the sp ectrum to b e very in

as the blue luminosity coming from the RC Fig

sensitive to the radiation intensity Boulanger et al

this explains the constant F F ratio observed in our

sample It results from this that even over the large range

Discussion

in HII region sizes and densities seen along the Hubble se

quence what we observe in disks is mainly the emission

Integrated versus lo cal quantities

from photo disso ciation regions and the lling factor by

HII regions is always comparatively small

The ab ove calibration derived from integrated disk uxes

Although midIR emission can in principle originate

should not b e applied lo cally in small regions of disks for

from regions where old stars dominate when integrated

several reasons

throughout spiral disks the emission in b oth bandpasses

In the diuse interstellar medium far away from any

traces young stars and the heating provided by a more

starforming site a substantial part of UIB carrier

evolved p opulation app ears negligible Otherwise the

heating maybeprovided by optical photons since it

F H ratio would increase with decreasing SFR sur

has b een demonstrated for instance in reection neb

face density as the imp ortance of the diuse interstellar

ul that this typ e of heating can b e ecient Uchida

medium relative to star forming regions is then higher

et al Lo oking at quantities integrated over large

this is not observed and neither is a variation of F H

spatial scales weaverage in our b eam all the stel

with Hubble typ e

lar p opulations contributing to the excitation of dust

An alternative cause of the observed correlation could

Since dust heating by ionizing photons is much more

be a much more indirect link b etween star formation and

ecientthanby nearultraviolet or optical photons a

dust emission b oth of them b eing causally asso ciated with

small p opulation of massive stars mixed with a large

the molecular gas phase dust is mixed with gas and stars

p opulation of lowmass stars can still dominate dust

form out of molecular clouds and these two links then

heating as so on as it exceeds a certain threshold still

simulate a direct connection b etween the presence of mas

to b e determined It is clear that this could no longer

sive stars and dust excitation Wehave no means of de

b e the case in selected regions of galaxies

ciding which scenario is the more likely and they would

Close to HII regions and in some giant star formation

observationally b e extremely dicult to test in particular

complexes the F F color is observed to rise ab ove

b ecause molecular gas mass estimations are not accurate

the mean value in disks see for instance NGC in

enough Nevertheless the presence of molecular gas is a

the Atlas revealing a thermo dynamical state dierent

prerequisite but certainly not a sucient condition for star

from that in disks for the sp ecies emitting at these two

formation so that the link b etween the two is not more

wavelengths This m excess clearly breaks down

direct than the link b etween young stars and dust emis

the symmetry of the and m uxes and then most

sion

likely the validity of our calibration

Assuming a purely linear correlation b etween UIB and

t fraction of ionizing photons is able to es A signican

H emission as indicated in Fig by the dashed line

cap e HII regions and p ossibly to propagate to very

and using the HSFR calibration of Kennicutt a

large distances In such a conguration a physical link

leads to the following scalings

between young stars and dust heating can b e estab

SFR M yr L m L lished only using integrated uxes

UIB bol

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

In Fig we plot the correlation b etween the size To supp ort the last caveat we note that Beckman et

normalized total farIR and H uxes for galaxies in our al prop ose from their study of H luminosity func

sample Given the very p o or spatial resolution of current tions of HII regions that the estimated ionizing luminosity

farIR instruments it is imp ossible to separate nuclear able to escap e the most luminous HII regions is more than

and disk contributions We see here that the correlation is sucient to ionize the warm diuse interstellar medium

worse than that obtained in Fig and in particular that see also Oey Kennicutt and references therein

the wellknown nonlinearity of the correlation is found in it is hence also able to heat dust to comparable distances

our sample to o a leastsquare t to our data gives a slop e If the idea that the diuse ionized gas is excited bythe

of The results obtained are highly dep endenton photons escaping from HII regions is conrmed then the

the adopted references for IRAS uxes among the various high H luminosity fractions attributable to this diuse

catalogs Moshir et al Rice et al Soifer et al medium according to eg Ferguson et al

Helou et al Sanders et al Rush et al imply that a correct assessment of star formation activity

Thuan Sauvage PSC whichmay partially cannot b e made if restricted to limited regions neither

account for the disp ersion However wehave tried to iden using recombination lines nor using the infrared emission

tify the most reliable uxes by optimizing the agreement

Vogler et al havelooked at the relationship

between our and m uxes and IRAS m uxes

between H and m uxes lo cally inside the disk of

whose bandpass overlaps with b oth ISOCAM bandpasses

NGC using resolution elements of p c at

Lo oking for the origin of this nonlinearitywend

our adopted distance They nd a very large disp ersion

that it is mainly intro duced in our sample by prominent

mostly at low ux levels so that the correlation that we

circumnuclear regions Indeed when the fraction of the

rep ort here breaks down at kp cscales It is however di

m ux contributed bycentral regions increases from

cult to disentangle in the observed scatter the role played

nearly to nearly the mean F F H ratio rises

FIR byvariable extinction in the H line spatially and also

from to The increase of F F His

as a function of brightness from a physical decorrelation

FIR

also connected with the app earance of high F F colors

whichhavebeenshown in Pap er I to originate in central

MidIR versus farIR emission as tracers of star

regions of galaxies whose total midinfrared emission is

formation

dominated bya central starburst We can further check

this nding by estimating the fraction of farIR emission

In Sect restricting ourselves to spiral disks wehave

arising from the disk alone and pro ducing an analog of

found that midIR luminosities can b e used to trace the

Fig for the farIR Since no spatiallyresolved measure

level of star formation As mentioned in Sect n umerous

ments are available wehave to rely on disk ux fractions

attempts to use the farIR luminosity as a tracer of star

measured at m and assume that they can accountfor

formation have already b een presented see eg Devereux

the true fractions in the farIR Given that total m

Young Kennicutt a and references therein

and farIR uxes are tightly correlated this assumption is

It is thus worthwhile to examine whether the tracer we

sensible This empirical correction succeeds in linearizing

prop ose here presents some signicantadvantage over the

the farIRH relationship and reducing the scatter the

farIR one The main problem with the farIR luminosity

least squares and least absolute deviation ts give a slop e

is that the heating radiation required for grains to emit in

of resp ectively and This demonstrates that

that wavelength range can easily b e provided by relatively

it is the circumnuclear contribution which creates the non

old stars of yr or more Buat Xu and thus the

linearities observed in previous attempts to correlate the

information derived on the SFR is dierent from that ob

farinfrared and H uxes

tained from H In other terms when plotting farIR ver

To further supp ort this view wehavelooked at corre

sus H data there is a hidden variable which corresp onds

lations b etween total midinfrared and H uxes adding

to the amount of energy provided by nonionizing stars It

the two galaxies whose midinfrared and H emission is

is this hidden variable which has previously b een deemed

highly concentrated NGC and Fig shows the

resp onsible of the signicant nonlinearity of the farIR

result at m compared with what wehave previously

H correlation eg LonsdalePersson Helou

obtained in disks Owing to the fact that H data were

Fig shows that the twointegrated farIR and m

found for only few centrally dominated galaxies with a

sizenormalized uxes are tightly and linearly correlated

m excess the relationship for total uxes is not much

Consequently there cannot b e ma jor dierences in the

dierent from that for disk uxes Ho wever the ts in

heating source for the small grains resp onsible for the

dicate that the disp ersion is already higher and that we

m emission and the large grains emitting in the far

intro duced a nonlinearity As the extinction in circumnu

infrared If we restrict ourselves to galaxies with small

clear regions is exp ected to b e higher than in the average

circumnuclear contribution in Fig those for whichthe

two symb ols used are close to each other this suggests

Using a large sample drawn from the H catalogues men

that in galaxies which are dominated by their disk the

tioned in Sect and applying a robust estimation metho d to

farinfrared emission is an equivalently go o d star forma

reduce the eects of outlying p oints since a signicant scatter

m emission is present we obtain F H tion tracer as the and FIR

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

disk the interpretation of the cause for the nonlinearity Summary and conclusions

is not straightforward We p ostp one the detailed study of

midinfrared emission and other star formation estimates

Wehave seen that midinfrared uxes can b e considered

in circumnuclear and starburst regions Forster Schreib er

reliable tracers of star formation in relatively quiescent

Roussel in preparation and outline that the relation

environments such as spiral galactic disks There mid

ship found here is strictly valid only in disks and cannot

infrared sp ectra b etween and m are dominated by

b e extrap olated easily

aromatic bands whicharemuch more tightly linked with

the radiation from young stars than with the blueband

radiation to which stars of all masses and ages contribute

The calibration in terms of SFR that we prop ose de

Applicability at high redshift

p ends of course on the adopted IMF but most sensitively

on the extinction correction applied to H uxes The

disp ersion in H uxes with resp ect to the linear corre

In cosmological surveys with the HST SFRs are mea

lation is dex in Fig muchlower than the extinc

sured with uxes that corresp ond to ultraviolet wave

tion correction of dex We nd a bivariate disp ersion

lengths in the restframe of distant galaxies For this rea

around the linear correlation of a factor of and a

son wehave tried to compare the SFRs deduced from mid

onedimensional disp ersion of a factor of b oth at

infrared uxes using Eq with the SFRs derived from ul

and m Despite this large scatter it is meaningful to

traviolet uxes at A using the calibration given in

apply our calibration to large samples b ecause the size

Kennicutt a The data come from Rifatto et al

normalized SFR showsamuch larger range it varies by

Deharveng et al and Bell Kennicutt

more than a factor of across the present sample

whose lters are not centered exactly at A

but at and A and the sample was limited to

Nevertheless one must b e aware of several limitations

galaxies either with a disk ux fraction ab ove or with

Because dust grains can b e heated by the radiation from

F F resp ectively and ob jets b oth crite

HII regions at large distances from them and b ecause

ria leading to the same conclusion UV uxes haveonly

dust in some diuse regions can b e predominantly heated

b een corrected for Galactic extinction in the same wayas

by old stellar p opulations the relationship b etween mid

H and the internal extinction has b een estimated bythe

infrared uxes and star formation rates is certainly much

requirement that the or m and ultraviolet SFR val

more complex lo cally than when considering integrated

ues agree for each spiral Resulting UV absorptions range

uxes

between and mag and their median value amounts

For distant ob jects detected in surveys the only avail

to mag We recall that our calibration assumes an

able information consists of uxes integrated over the

H absorption of mag and that changing AHwould

whole galaxywhich can b e dominated by the central re

change A by the same amount

gions In this case the link b etween star formation and

midinfrared emission can also b e completely dierent

Indeed weshow that considering global uxes in the far We can compare these numb ers with those obtained in

infrared as well as at m intro duces a nonlinearity the highredshift sample of Flores et al z

in the correlation with H uxes This may b e due to a who have p erformed a similar comparison b etween

combination of several eects a thermo dynamical state of star formation rates derived from ultraviolet and infrared

dust grains in central regions dierent from that in disks observations They have extrap olated ultraviolet uxes at

a greater extinction aecting the H line a dominant con A from sp ectral energy distributions ab ove A

tribution from p oststarburst p opulations of the bulge to using a grid of sp ectrophotometric evolutionary mo dels

dust heating in the absence of signicant star formation and have also estimated total infrared uxes from to

Our sample comprises only spiral galaxies whose metal m from midinfrared and radio uxes tted by a set

licity is thought to b e near solar while metaldeciency of templates Assuming the same IMF as in the present

mostly seen in blue compact and dwarf irregular galax work and the SFR calibrations of Kennicutt a b oth

ies can alter the dust comp osition and is likely to de in terms of UV and IR uxes Flores et al de

plete carb onaceous grains which tends to lower the mid rive extinctions in the range A mag This

infrared emission for a given radiation eld Sauvage et al translates into A using the extinction

Boselli et al The aromatic bands are often curves of Cardelli et al Absorption estimates in

absentorvery weak in dwarf galaxies due to the ab ove ef our sample are thus consistent well within the uncertain

fect combined with their destruction by the farultraviolet ties with those in the sample of Flores et al Hence

radiation very p ervasiveinlowmetallicityenvironments although absorption estimates can b e awed by metallic

Madden ity eects in young galaxies if they deplete preferentially

In extremely activeenvironments with normal metal small carb onaceous grains emitting in the midinfrared

licity the aromatic band carriers can also b e destroyed there is no hint of a signicant increase of optical depths

or exp erience chemical transformations but quantitative with redshift which implies that the calibration wegive

estimations of these eects are yet unavailable likely remains valid in more distant galaxies 8 H. Roussel et al.: Relationship between SFRs and MIR emission in galactic disks

However, this relationship between mid-IR fluxes and Buat V. & Xu C. 1996, A&A 306, 61 SFRs that strictly holds only in normal disks can be useful Cardelli J.A., Clayton G.C. & Mathis J.S., 1989, ApJ 345, to interpret surveys made in the two filters LW3 (15 µm) 245 and LW2 (7 µm). For galaxies at high redshifts (z  1.2), Cesarsky D., Lequeux J., Abergel A. et al., 1996, A&A the LW2 rest frame emission is shifted to the LW3 band- 315, L309 pass. Hence, the calibration given here must provide a Coulais A. & Abergel A., 2000, A&AS 141, 533 lower limit for the true SFR since, for galaxies with greater Crocker D.A., Baugus P.D. & Buta R., 1996, ApJS 105, star formation activity than in the present sample, the en- 353 ergy redistribution favors the LW3 band, as more energy is Dale D.A., Silbermann N.A., Helou G. et al., 2000, AJ reradiated by VSGs. Indeed, Boulanger et al. (1998b) have 120, 583 presented observations in resolved Galactic regions (a dif- Deharveng J.-M., Sasseen T.P., Buat V. et al., 1994, A&A fuse cloud and four photodissociation regions) which show 289, 715 that the emission in UIBs tends to rise with the ultraviolet D´esert F.-X., Boulanger F. & Puget J.L., 1990, A&A 237, energy density, linearly at low energy densities and more 215 slowly at higher values. The threshold for this transition, Devereux N.A. & Young J.S., 1990, ApJ 350, L25 above 103 times the energy density in the solar neighbor- Engargiola G., 1991, ApJS 76, 875 hood, is uncertain due to dilution effects. However, the Falcke H., Wilson A.S. & Simpson C., 1998, ApJ 502, 199 same type of behavior must hold in integrated galaxies. Feinstein C., 1997, ApJS 112, 29 Dealing with integrated fluxes, a simple validity crite- Ferguson A.M.N., Wyse R.F.G., Gallagher J.S. & Hunter rion of the formula presented here would be a rest-frame D.A., 1996, AJ 111, 2265 color F15/F7  1, i.e. the contribution from the central Flores H., Hammer F., Thuan T.X. et al., 1999, ApJ 517, concentration to the total emission should be low or should 148 arise from heating by a disk-like (non-starburst) stellar F¨orster-Schreiber N.M. & Roussel H., 2001, in preparation population. Garc´ıa-Barreto J.A., Franco J., Carrillo R., Venegas S. & Escalante-Ram´ırez B., 1996, RMxAA 32, 89 Acknowledgements. We warmly thank Antonio Garc´ıa- Giard M., Pajot F., Lamarre J.M., Serra G. & Caux E., Barreto, Thaisa Storchi-Bergmann, Magnus Naslund, Carlos 1989, A&A 215, 92 Feinstein, Michael Regan, Stuart Ryder and Fran¸cois Viallefond for freely providing their Hα maps. We wish to Greenawalt B., Walterbos R.A.M., Thilker D. et al., 1998, thank Suzanne Madden for taking part in the improvement of ApJ 506, 135 the manuscript and Herv´e Aussel for a discussion about high Hameed S. & Devereux N., 1999, AJ 118, 730 redshift studies of star formation. Helou G., 2000, in Infrared astronomy: today and to- The ISOCAM data presented in this paper were analyzed morrow, Les Houches Summer School Aug. 1998, ed. using and adapting the CIA package, a joint development by Springer-Verlag, 337 the ESA Astrophysics Division and the ISOCAM Consortium Helou G., Khan I.R., Malek L. & Boehmer L., 1988, ApJS (led by the PI C. Cesarsky, Direction des Sciences de la 68, 151 Mati`ere, C.E.A., France). Helou G., 1986, ApJ 311, L33 Ho L.C., Filippenko A.V. & Sargent W.L.W., 1997, ApJS References 112, 315 Hony S., van Kerckhoven C., Peeters E. et al., 2001, A&A Armus L., Heckman T.M. & Miley G.K., 1990, ApJ 364, 370, 1030 471 Keel W.C., 1983, ApJS 52, 229 Beckman J.E., Rozas M., Zurita A., Watson R.A. & Kennicutt R.C., 1998a, ARA&A 36, 189 Knapen J.H., 2000, AJ 119, 2728 Kennicutt R.C., 1998b, ApJ 498, 541 Bell E.F. & Kennicutt R.C., 2001, ApJ 548, 681 Kennicutt R.C., Tamblyn P. & Congdon C.W., 1994, ApJ Binette L., Magris C.G., Stasin´ska G., Bruzual A.G., 1994, 435, 22 A&A 292, 13 Kennicutt R.C., Keel W.C., van der Hulst J.M., Hummel Boselli A., Lequeux J., Sauvage M. et al., 1998, A&A 335, E. & Roettiger K.A., 1987, AJ 93, 1011 53 Kennicutt R.C., 1983, ApJ 272, 54 Boulade O., Sauvage M., Altieri B. et al., 1996, A&A 315, Kennicutt R.C. & Kent S.M., 1983, AJ 88, 1094 L85 L´eger A. & Puget J.L., 1984, A&A 137, L5 Boulanger F., Abergel A., Bernard J.P. et al., 1998b, ASP Lemke D., Mattila K., Lehtinen K. et al., 1998, A&A 331, Conf. Series 132, 15 742 Boulanger F., Boissel P., Cesarsky D. & Ryter C., 1998a, Lindblad P.O., 1999, A&AR 9, 221 A&A 339, 194 Lonsdale-Persson C.J. & Helou G., 1987, ApJ 314, 513 Boulanger F., Reach W.T., Abergel A. et al., 1996, A&A Madden S.C., 2000, New Astr. Review 44, 249 315, L325 Mattila K., Lemke D., Haikala L.K. et al., 1996, A&A 315, Brand P.W.J.L., Coulson I.M. & Zealey W.J., 1981, L353 MNRAS 195, 353 H. Roussel et al.: Relationship between SFRs and MIR emission in galactic disks 9

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H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

Fig Comparison of total sizenormalized uxes in the blue

band and at m The dashed line is the linear correlation and

the b est least squares t and b est least absolute deviation t

are shown as dotdashed and solid lines The tted slop es are

and with a condence interval and a

linear correlation co ecient of Using m uxes instead

of m uxes or Hband uxes for the stellar emission leads

to very similar results Restricting the sample to the galaxies

present in Fig or to the galaxies dominated by disk emission

at mby more than also pro duces similarly disp ersed

distribution s with tted slop es always ab ove

Fig Prop ortionali tybetween farinfrared and midinfrared

sizenormalized uxes FarIR uxes are a combination of

and m IRAS uxes as dened in Helou et al Filled

Fig Relationship b etween sizenormalized uxes of H and

circles represent total uxes and op en circles show the eect of

a F b F to m to m The dashed line is

taking into account only bare disks as dened in Pap er I this

the linear correlation The b est least squares t and least ab

was p ossible only at m since galaxies are not resolved by

solute deviation t are shown as dotdashed and solid lines



IRAS The b est leastsquares t gives a slop e of

 NGC and NGC were excluded b ecause their disk

ata condence level with a correlation co ecient of

b was not completely mapp ed at and m Excluding in

The relationship with m uxes is similar but more disp ersed

the two galaxies with susp ected signicant contribution from



at a condence and with a slightly higher slop e

F F  stellar emission at m b ecause of their low ratios



level

VCC and NGC the tted slop es b ecome and

instead of and c shows that F F is con



stant with a go o d approximation in disks Flux density ratios Jy units can b e obtained applying a factor dex

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

Fig Relationship between total farIR and H size

normalized uxes The dashed line represents the linear cor

relation the dotdashed line the least squares ts and the

solid line the least absolute deviation t with resp ectiveslopes



 interval and



total Fig  Same as Fig with sizenormalized m and

H uxes sup erimp osed shown as empty circles The solid and

dotdashed lines corresp ond to the ts on disk uxes of Fig

The names of the most strongly deviating galaxies and the nu

 merical results of the ts on total uxes are indicated At m

only NGC and tend to stray from the relationshi p found in disks

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

Table Photometric data Galaxies are named according to the VCC catalog for the Virgo program and according to the

NGC catalog for other programs

a b a c c d d e

name H NII D H NII f f F F refs

TOT C CEN C C DISK DISK

log W m arcsec log W m mJy

N a

N

N

N

N

N

N

N a

N

N

N

N

N b

N

N b

N

N

V N

V N

V N

V N

V N b

V N

V N

V N

V N

V N

V N

V N

V N

V N

V N

V N

V N a

N

N

N a

N

N

N

N

N

N a

N

N

N a

a

corrected for Galactic extinction using the RC blue absorptions and the extinction curve of Cardelli et

al Total pure H uxes available for N N N and N were made homogeneous

with H NII uxes by applying a factor

b

diameter ap erture or slit dimensions of the measurement used to remove a central H contribution

c

approximate fractions of central midinfrared uxes inside the same ap erture as used for H corrected

for dilution eects

d

midinfrared uxes after removal of the central contribution matched to the H ap erture

H Roussel et al Relationshi p b etween SFRs and MIR emission in galactic disks

e

Reference co des for the total H ux and the central H ux

Armus et al Cro cker et al Engargiola Falcke et al

map provided byCFeinstein map provided by JA GarcaBarreto map taken from the

electronic edition of Greenawalt et al Hameed Devereux Ho et al

Keel a Kennicutt Kent b Kennicutt et al map provided by

M Naslund Pogge map provided by M Regan Romanishin map

provided by SD Ryder Ryder Dopita Sheth et al Stauer

map provided by T StorchiBergmann VeronCettyVeron map provided byF

Viallefond Wang et al Young et al

The correction for stellar absorption of Ho et al was cancelled using their data



close system we used the total H ux listed byYoung et al weighted by the two contributions

inside a smaller ap erture derived from Kennicutt et al



H NII ux estimated from a pure H ux

V. Formation d’´etoiles et rayonnement de la poussi`ere 133

V.3 Pr´ecisions sur les cons´equences pour l’infrarouge lointain

Nous venons de voir que la r´esolution angulaire dont nous disposons en infrarouge moyen, qui nous permet de s´eparer les r´egions galactiques centrales des disques, nous fournit par l`a- mˆeme des renseignements sur les variations spatiales probables de l’´emission en infrarouge loin- tain. Rappelons que les flux totauxa15 ` µm et en infrarouge lointain sont corr´el´es lin´eairement dans l’´echantillon de galaxies ´etudi´e. Sellgren et al. (1990), dans leur ´etude de n´ebuleuses par r´eflexion, avaient d´ej`anot´equeI12/Ibol FIR (le rapport de la puissance ´emise `a12µm sur la puis- sance bolom´etrique estim´ee en infrarouge lointain) ne montrait aucune variation syst´ematique en fonction de la temp´erature de l’´etoile excitatrice. Sauvage & Thuan (1994) ont ´egalement mis en ´evidence une ´etroite corr´elation entre les fluxa12et100 ` µmdansun´echantillon de galaxies spirales s´electionn´ees optiquement. Cette bonne ad´equation de l’´emission en infrarouge moyen avec l’´emission en infrarouge lointain sugg`ere que leur distribution spatiale doit ˆetre similaire.

La figure V.11 indique sans ambigu¨ıt´equelesr´egions centrales de galaxies qui ont des couleurs F15/F7 ´elev´ees et dominent l’´emission en infrarouge moyen (voir le chapitre VI pour la d´emonstration que des couleurs globales ´elev´ees sont dues `a des couleurs centrales ´elev´ees) sont responsables d’une forte augmentation du rapport LFIR/LHα. Cependant, la raison exacte de cette augmentation n’est pas ´elucid´ee : ce pourrait ˆetre soit `a cause d’une plus forte extinction en Hα dans les r´egions centrales, soit par un chauffage additionnel de la poussi`ere par des popu- lations stellaires vieilles r´esidant dans le bulbe, et suffisamment denses pour fournir une densit´e de rayonnement ´elev´ee (Sauvage & Thuan 1994), soit que l’efficacit´e de conversion des photons ionisants en photons infrarouges soit accrue. J’examine ci-dessous le lien entre la formation d’´etoiles et l’´emission en infrarouge moyen dans les r´egions centrales de quelques galaxies de l’´echantillon. 134

Figure V.11: Rapport des puissances ´emises en infrarouge lointain et dans les raies (Hα +[NII]) en fonction de la fraction du flux totala15 ` µm provenant des r´egions circum- nucl´eaires (en haut) et de la couleur F15/F7 totale (en bas). La valeur moyenne de l’ordonn´ee est d’environ 340 pour les galaxies domin´ees par leurs r´egions centrales (fCNR(15 µm)> 0.5)et 140 pour les autres. La galaxie dont le nom est indiqu´e, VCC 836, est une galaxie de Seyfert dont les activit´es stellaire et non-stellaire contribuentaparts´ ` equivalentesal’´ ` emission infrarouge. V. Formation d’´etoiles et rayonnement de la poussi`ere 135

V.4 Dans les r´egions circumnucl´eaires

L’´etude de l’´evolution de la relation entre l’´emission de la poussi`ere en infrarouge moyen et l’activit´e de formation d’´etoiles, lorsqu’on passe des conditions pr´evalant dans les disques spiraux aux starbursts mod´er´es des r´egions circumnucl´eaires, a b´en´efici´ed’unr´esultat obtenu par Natascha F¨orster-Schreiber (F¨orster-Schreiber et al. 2001). Dans trois galaxiesa ` sursauts de formation d’´etoiles (M 82, NGC 253 et NGC 1808) – beaucoup plus actives que les galaxies de mon ´echantillon – observ´ees par ISOCAM dans son mode de spectro-imagerie, elle a ´etabli l’existence d’une bonne corr´elation entre les flux mesur´es `a15µm dans une bande ´etroite, et les flux dans la raie [ArII]`a6.99µm, qui doivent ˆetre proportionnels aux flux de photons ionisants dans les conditions d’excitation des r´egions HII des trois galaxies. Comme les r´egions circum- nucl´eaires des galaxies de mon ´echantillon ont des activit´es de formation d’´etoiles interm´ediaires entre les disques spiraux et ces trois galaxies starbursts, la juxtaposition des donn´ees disponibles nous permet d’explorer la relation entre ´emission de la poussi`ere et taux de formation d’´etoiles sur plus de quatre ordres de grandeur.

Pour tenter de r´esoudre la difficult´e d’estimer l’extinction moyenne dans les r´egions circum- nucl´eaires, j’ai cherch´e des donn´ees dans des raies de recombinaison de l’hydrog`ene en infrarouge proche. En effet, la raie Brγ, par exemple, `a2.17µm, subit une extinction environ 7 fois plus faible que la raie Hα,`a 6563 A˚ (en utilisant la loi d’extinction de Cardelli et al. 1989), alors que le rapport des extinctions dans les raies Hβ et Hα est seulement d’environ 1.4, en utilisant la mˆeme loi. Nous disposons donc en utilisant le d´ecr´ement Brγ/Hα d’une meilleure estimation de l’extinction qu’`apartirdud´ecr´ement Hα/Hβ. L’inconv´enient de cette m´ethode est la raret´edes donn´ees dans les raies de Paschen ou Brackett (qui requi`erent une grande sensibilit´e), et qui plus est dans des ouvertures qui ne soient pas trop petites, de fa¸con `apouvoirr´ealiser des mesures raisonnables sur les cartesa7et15 ` µm. Par exemple, des donn´ees Brγ pour NGC 613, 1672 et 986 n’ont pu ˆetre utilis´ees parce que les ouvertures, inf´erieures `a6, ne sont pas appropri´ees `a notre r´esolution angulaire.

J’ai aussi essay´e d’estimer l’extinction dans la raie Hα en utilisant le continuum radio `a diff´erentes fr´equences, de mani`ere `apouvoirens´eparer les composantes thermique et syn- chrotron. L’indice spectral de l’emission libre-libre a ´et´e suppos´e valoir −0.1:

    S ν −0.1 ν anth ν f ν − f ν S = th( 0) ν +[1 th( 0)] ν (V.1) ν0 0 0 o`u ν0 est une fr´equence de r´ef´erence, fth la fraction thermique de l’´emission et anth l’indice spectral de l’´emission non-thermique. Niklas et al. (1997) trouvent un indice moyen anth ≈−0.8 pour l’´emission radio int´egr´ee des galaxies, dans une fourchette [−0.4; −1.2]. Si la composante thermique est connue, l’extinction en Hα peut ˆetre d´eduite de la th´eorie de la recombinaison en faisant l’hypoth`ese d’une certaine temp´erature ´electronique. Cette m´ethode exige de disposer d’observations au moinsatroisfr´ ` equences (dans des ouvertures compatibles), et si possible allant au-del`ade10GHz,c’est`a dire dans un domaine de fr´equence o`ul’´emission ne soit pas enti`erement domin´ee par la composante synchrotron. La fraction thermique et l’indice spectral non-thermique sont par ailleurs tr`es sensibles `a de petites variations des flux. Les donn´ees que j’ai pu trouver pour une dizaine de galaxies sont trop disparates et trop impr´ecises pour estimer 136

Tableau V.1: Photom´etrie des r´egions circumnucl´eaires. Les flux en infrarouge moyen ont ´et´e mesur´es sur les images trait´ees avec la proc´edure analogue de CLEAN.

nom DC log Brγ log Hα A(Brγ)A(Hα) F15 F7 ref.(Brγ)ref.(Hα) (arcsec) (W.m−2)(W.m−2) (mJy) (mJy) N1097a ≈ 30. ≥−16.16 -14.59 ≥ 0.19 ≥ 1.30 1562 ± 94 1134 ± 63 (K) (H) N1365b 23.5 -15.71 -14.55 0.36 2.50 2624 ± 366 1564 ± 244 (P88) (N, O) N4691c 24.5 -16.43 -15.11 0.29 2.03 493 ± 61 329 ± 39 (P90) (G) N5236d 23.5 -15.71 -14.19 0.21 1.45 ≥ 2730 ± 162 ≥ 2054 ± 156 (P88) (R) N7552e ≈ 12. -15.96 -14.71 0.32 2.23 ≥ 1986 ± 128 1049 ± 125 (S) (F) N4102f 23.5 -16.23 ≥ 1371 ± 557 ≥ 444 ± 111 (P88) N6946g 23.5 -16.21 ≥ 1520 ± 716 ≥ 822 ± 263 (P88) N7771h ≈ 10. ≥−17.26 208 ± 61 162 ± 23 (RK)

aNGC 1097 : L’ouverture, qui n’est pas sp´ecifi´ee clairement, est approximativement la taille de l’anneau. Le flux Brγ est sans doute une limite inf´erieurea ` cause des vitesses radiales ´elev´ees dans l’anneau et de l’´etroitesse du filtre utilis´e. Le flux Hα peut ˆetre sur´evalu´e`a cause de l’emploi de la relation Hα = 0.75 (Hα +[NII]). bNGC 1365 : L’ouverture effective a ´et´ecalcul´√ ee `a partir de la largeura ` mi-hauteur du faisceau, en supposant celui-ci gaussien (DC =FWHM/ ln 2 ). cNGC 4691 : L’ouverture effective a ´et´ecalcul´ee de la mˆeme mani`ere que pour NGC 1365. dNGC 5236 : Le noyau est satur´e`a7et15µm. eNGC 7552 : mˆeme remarque sur l’ouverture que pour NGC 1097. Le noyau est l´eg`erement satur´e`a 15 µm. Le flux Hα a´et´eestim´e par la relation Hα =0.75 (Hα +[NII]). fNGC 4102 : Le noyau est l´eg`erement satur´e`a7et15µm. gNGC 6946 : Le noyau est satur´e`a7et15µm. hNGC 7771 : mˆemes remarques sur l’ouverture et le flux Brγ que pour NGC 1097. r´ef´erences : (F) carte Hα de C. Feinstein ; (G) carte Hα d’A. Garc´ıa-Barreto et Garc´ıa-Barreto et al. (1995) ; (H) Hummel et al. (1987) ; (K) Kotilainen et al. (2000) ; (N) carte Hα de M. Naslund ; (O) Osmer et al. (1974) ; (P90) Puxley et al. (1990) ; (P88) Puxley et al. (1988) ; (RK) Reunanen et al. (2000) ; (R) carte Hα de S. Ryder ; (S) Schinnerer et al. (1997). l’extinctiona ` mieux qu’un facteur 2 pr`es, si l’on n’est pas r´esolument optimiste. Je m’en suis donc tenuea ` l’utilisation du d´ecr´ement Brγ/Hα.

L’extinction dans la raie Hα est obtenue par :       α α H H − . A α − 1 log γ =log γ 0 4 (H ) 1 A α /A γ (V.2) Br obs Br intr (H ) (Br ) o`u le rapport intrins`eque (Hα/Brγ)intr est de 103.8 pour des conditions standard dans les r´egions HII (Kennicutt 1998a) et A(Hα)/A(Brγ)=7.03 selon la loi d’extinction de Cardelli et al. (1989).

En utilisant les cartes Hα dont je dispose pour quelques galaxies, il est possible d’adapter l’ouverture photom´etrique aux donn´ees trouv´ees en infrarouge proche. Lorsque la bande passante inclut les raies [NII], j’ai utilis´elarelationHα =0.75 (Hα +[NII]) donn´ee par Kennicutt. Cependant, les rapports [NII]/Hα sont en g´en´eral plus ´elev´es et plus variables dans les r´egions centrales des galaxies, en cas d’excitation par des chocs ou un rayonnement ultraviolet dur. Les donn´ees sont rassembl´ees dans le tableau V.1.

La figure V.12 montre la mise en commun des donn´ees des disques et des donn´ees des r´egions V. Formation d’´etoiles et rayonnement de la poussi`ere 137 centrales des galaxies spirales de mon ´echantillon. Pour les disques, les quantit´es trac´ees sont les flux normalis´es par la surface du disque optique (voir le chapitre V.2.1), tandis que pour les r´egions centrales, ce sont de vraies brillances de surface. Bien que ces quantit´es ne soient pas strictement ´equivalentes, changer la normalisation revienta ` faire glisser les points le long d’une droite de pente 1. Malgr´e les incertitudes assez substantielles, il semble que le prolongement soit meilleura7 ` µmqu’`a15µm. L’´emission des r´egions circumnucl´eaires doit n´ecessairement se comporter diff´eremment `a ces deux longueurs d’onde, puisque leurs couleurs F15/F7 sont plus ´elev´ees et plus variables que celles des disques. Dans les centres des galaxies, le spectre des bandes d’´emission aromatiques reste tr`es semblable `a celui des disques. La diff´erence essentielle est l’apparition du continuum thermique ´emis par les tr`es petits grains au-del`ade10µm, qui se superpose dans la bande passante du filtrea15 ` µm aux bandes aromatiques d´ej`apr´esentes dans les disques.

On peut donc interpr´eter la brisure dans la relation Hα-15 µm comme une transition entre des conditions o`u une seule phase de poussi`ere domine (les porteurs des bandes aromatiques), et des conditions o`u une autre phase contribue significativemental’´ ` emission ou mˆeme la domine (les tr`es petits grains). On aurait pu s’attendre par ailleurs `a ce que la relation Hα-7 µm s’infl´echisse pour des grandes densit´es de photons ionisants. En effet, on a observ´e que les porteurs des bandes aromatiques sont d´etruits `a l’int´erieur des r´egions HII (par exemple Giard et al. 1994). On peut supposer que lorsque la densit´eder´egions HII augmente, les enveloppes de photodissociation se recouvrent partiellement, provoquant une augmentation du facteur de remplissage du milieu interstellaire par les zones ionis´ees. Comme l’´emission des bandes aromatiques provient surtout des zones de photodissociation et des surfaces des nuages mol´eculaires (Cesarsky et al. 1996), cela am`enerait une saturation de l’´emission `a7µm pour des densit´es de formation d’´etoiles plus grandes. Il semble donc que dans les centres des galaxies ´etudi´ees, on n’ait pas atteint ce r´egime. Qu’en est-il pour des galaxies starbursts ?

Natascha F¨orster-Schreiber m’a communiqu´e ses mesures sur les spectres ISOCAM de M 82, NGC 253 et NGC 1808. Pour ces galaxies, l’estimation du flux de photons ionisants, et donc du taux de formation d’´etoiles, est fournie par l’observation de la raie interdite [ArII]`a6.99µm. En supposant que la m´etallicit´e est solaire ([Ar/H] = 3.98 10−6), que tous les atomes d’argon sont −3 ionis´es une fois, que la densit´eetlatemp´erature ´electroniques sont de l’ordre de ne = 300 cm et Te = 5000 K (F¨orster-Schreiber et al. 2001), les ´emissivit´es dans les raies Hα et [ArII]6.99µm sont  (Hα)=6.69 10−25 erg.s−1.cm−3 et  ([ArII]) = 1.55 10−20 erg.s−1.cm−3. On obtient alors le rapport de flux intrins`eque

N + n  α F α /F (H ) e (H ) . . (H ) ([ArII]) = + =1084 (V.3) N(Ar ) ne  ([ArII] 6.99)

A partir des spectres, qui vont de 5a ` 16-17 µm environ, il est possible de simuler les flux en bande largea7 ` µm (5–8.5 µm), mais pas `a15µm (12–18 µm). Pour estimer le niveau du continuum thermique des tr`es petits grains, Natascha a mesur´e les flux F15 ct dans l’intervalle 14.81–15.19 µm, dans une zone qui n’est en principe pas contamin´ee par des bandes d’´emission ou des raies ioniques, et qui n’est pas trop affect´ee par la non-stabilisation du d´ebut des observations (aux plus grandes longueurs d’onde). Il est possible de reproduire ces mesures pour les galaxies de mon ´echantillon observ´ees en mode de spectro-imagerie, c’est `a dire NGC 613, 1097, 1365, 138

Tableau V.2: Photom´etrie des r´egions circumnucl´eaires dans la bande passante 14.81–15.19 µm. Le facteur de correction de dilution appliqu´e est celui qui a ´et´ed´etermin´e`a15µm (bande pas- sante 12–18 µm) grˆace aux cartes trait´ees avec l’analogue de CLEAN. Il est donc sous-estim´esi la taille du pixel est 3 pourlescarteset6 pour les spectres, ce qui est le cas pour NGC 1097 et 5149 ; cependant, cela est pr´ef´erable `a l’emploi de la PSF (matrice de r´eponse `a une source ponctuelle). La barre d’erreur donn´ee inclut l’incertitude sur le niveau du fond zodiacal (voir l’Atlas). Pour les galaxies autres que NGC 1365, des fantˆomes dus aux r´eflexions `a l’int´erieur de l’instrument peuvent contribuer au flux (de 10a ` 20% `a7µm, d’apr`es la comparaison pho- tom´etrique des spectres avec les cartes qui figure dans l’Atlas).

nom DC fdil F15 ct log Hα (arcsec) (mJy) (W.m−2) N1097 ≈ 30. ≥ 1.24 ≥ 1432 ± 55 N1365 23.5 1.32 2639 ± 63 N5194a 65. ≥ 1.08 ≥ 1101 ± 138 -14.44 N5236 23.5 1.30 3942 ± 79

aNGC 5194 : Pour cette galaxie, aucune mesure dans la raie Brγ n’est disponible, mais l’extinction en Hα peut ˆetre grossi`erement estim´ee `a l’aide de flux radio mesur´es `a 49 cm, 2.8 cm, 2 cm et 1.2 cm dans une ouverture de 65 (Klein et al. 1984) : j’obtiens 1.2 ≤ A(Hα) ≤ 2. Le flux Hα (donn´e non corrig´ede l’extinction) a ´et´eestim´e par la relation Hα =0.75 (Hα +[NII]).

5194 (= M 51) et 5236 (= M 83). Les r´esultats, en utilisant la mˆeme ouverture que pour les flux dans la raie Brγ, sont indiqu´es dans le tableau V.2. Comme le flux Brγ trouv´e pour NGC 613 a ´et´emesur´e dans une ouverture trop petite (6×6, correspondanta ` un pixel d’ISOCAM), cette galaxie est ´ecart´ee. Pour les galaxies du tableau V.1 qui n’ont pas ´et´eobserv´ees en mode de spectro-imagerie, on peut raisonnablement estimer leur flux dans l’intervalle 14.81–15.19 µmen faisant l’hypoth`ese que le rapport F15 ct/F12−18 ne d´epend que de la couleur F15/F7 mesur´ee dans la mˆeme r´egion : c’est ce que j’ai fait en utilisant les spectres et cartes de NGC 1097 et 1365 (NGC 5236 est inutilisable parce que son noyau est assez fortement satur´e dans les cartes).

Le r´esultat de la comparaison des galaxies starburst avec les autres est montr´e en Fig. V.13. Les donn´ees des galaxies starbursts n’ont pas ´et´e corrig´ees de l’extinction, qui est assez sus- tantielle mˆeme `a ces longueurs d’onde (F¨orster-Schreiber et al. 2001), mais elle affecte de fa¸con similaire la raie [ArII] et le continuuma15 ` µm. Les r´egions circumnucl´eaires de NGC 1365, 5236 et 7552 chevauchent le domaine d’activit´edesr´egions de M 82, NGC 253 et NGC 1808 en- tourant le cœur du starburst. Pour ces r´egions circumnucl´eaires comme pour celles des autres spirales, les observations sont compatibles avec l’existence d’une relation lin´eaire entre le flux de photons ionisants et l’´emission des tr`es petits grains, sur plus de deux ordres de grandeur. Cependant, le d´ecrochement entre les disques et les r´egions centrales, d´ej`a visible en Fig. V.12 (o`u les mesures dans l’intervalle 12-18 µmm´elangent bandes aromatiques et continuum ther- mique), est beaucoup plus marqu´edanslefiltre´etroit centr´e`a15µm. On peut estimer que ce d´ecrochement se produit pour des densit´es de photons ionisants moyennes comprises entre 3. 1045 et 3. 1046 s−1.pc−2 (´etant entendu que les quantit´es trac´ees pour les disques de galaxies ne sont pas de vraies densit´es de surface et les sous-estiment).

La figure V.14 montre le prolongement de la figure V.12a7 ` µm par l’ajout des galaxies starbursts. Au vu de ces donn´ees, l’´emission des bandes aromatiques des r´egions circumnucl´eaires V. Formation d’´etoiles et rayonnement de la poussi`ere 139 des galaxies normales se comporte de la mˆeme mani`ere que celle des galaxies starbursts vis `a vis du rayonnement ionisant. On commence peut-ˆetre `aobserverlefl´echissement attendu dans les cœurs starbursts, `a des densit´es de photons ionisants sup´erieures `a1048 s−1.pc−2, mais l’effet est incertain et seulement de l’ordre d’un facteur deux. En excluant ces cœurs starbursts, la relation entre taux de formation d’´etoiles et ´emission des bandes aromatiques, calibr´ee sur les disques de galaxies, est cependant valable sur plus de quatre ordres de grandeurs.

Si l’on applique cette relation directement aux mesures de flux `a7µm des galaxies de l’´echantillon, on obtient les distributions de taux de formation d’´etoiles totaux, dans les r´egions centrales et dans les disques montr´ees en Fig. V.15 `a titre indicatif. La densit´edesurface moyenne de la formation d’´etoiles circumnucl´eaire peut aussi ˆetre estim´ee varier dans notre −1 −2 ´echantillon entre 0.03 et 1.8 M.an .kpc ; ces chiffres correspondent `a une moyenne sur des r´egions centrales ´etendues, seules accessibles `anotrer´esolution angulaire, mais on peut s’attendre `a ce que la formation d’´etoiles y soit tr`es inhomog`ene et localement plus intense.

En r´esum´e, nous avons obtenu une indication solide, et assez surprenante, que les massifs de bandes aromatiques entre 6 et 13 µm constituent un traceur quantitatif de la formation stellaire, dans les galaxies de m´etallicit´e proche de la valeur solaire, pour des activit´es aussi diverses que celles observ´ees dans les disques de galaxies spirales normales (mˆeme an´emiques) `a celles observ´ees dans les cœurs (d’une taille de l’ordre de 500 pc) de galaxies starbursts comme M 82. En ce qui concerne la bande d’´emission `a3.3µm, dont les porteurs sont probablement de mˆeme nature que ceux des bandes observ´ees entre 6 et 13 µm, il faut mentionner que Moorwood (1986) aremarqu´e une association entre cette bande et l’activit´e de formation d’´etoiles (dans des noyaux galactiques de type HII), et son absence dans les noyaux de Seyfert. Mizutani et al. (1989) ont par ailleurs observ´e des noyaux starbursts dans la bandea3.3 ` µm et dans la raie Brγ (en utilisant une ouverture de l’ordre de 6, avec une sensibilit´e tout juste suffisante), et trouvent les variations de ces deux observables compatibles avec une relation lin´eaire (en flux non normalis´es, W.m−2), sur un peu plus d’un ordre de grandeur, de l’activit´enucl´eaire de NGC 7552a ` celle de M 82 (qui font partie de leur ´echantillon). Nous avons montr´e que contrairementa ` une id´ee commun´ement accept´ee, les bandes aromatiques restent un traceur de formation d’´etoiles valable dans des environnements environ 103 fois moins actifs que les r´egions centrales de NGC 7552.

Le continuum thermique des tr`es petits grains `a15µmconstitue´egalement un bon indicateur de formation d’´etoiles, et de plus ne montrant aucune apparence de saturation aux grandes densit´es de flux de photons ionisants, mais ne peut ˆetre utilis´e pour des activit´es comparables ou inf´erieures `a celle du plateau central de M 51, car l’´emission est alors d´ecal´ee vers des longueurs d’onde plus grandes.

Ces r´esultats ont des implications int´eressantes pour l’utilisation des mesures r´ealis´ees dans les relev´es profonds de galaxies en infrarouge moyen, o`usonts´electionn´ees des galaxies `a formation d’´etoiles intense et des galaxies abritant un noyau de Seyfert actif. Pourvu que l’on sache faire la distinction entre ces deux types d’activit´e, qui tous deux se refl`etent dans l’´emission de la poussi`ere (avec des signatures spectrales diff´erentes, puisque les bandes aromatiques sont d´etruites `aproximit´e d’un noyau de Seyfert et remplac´ees par un continuum, comme montr´e par exemple par Genzel et al. 1998), les flux en infrarouge moyen fournissent une contrainte sur l’´evolution de la formation stellaire. 140

Il serait souhaitable d’´etendre le domaine de validit´eexplor´educˆot´e des starbursts plus intenses, et aussi pour des m´etallicit´es plus faibles. Notons que d’apr`es Rigopoulou et al. (1999), 12 pour celles des galaxies ultralumineuses en infrarouge (d´efinies par LFIR > 10 L)dontla puissance est estim´ee provenir principalement de la formation d’´etoiles, et non d’un noyau de Seyfert, les rapports des flux dans la bandea7.7 ` µm et en infrarouge lointain sont similairesa ` ceux des galaxies starbursts locales, seulement 1.4 fois plus petits en moyenne ; cette l´eg`ere baisse peut r´esulter d’une destruction partielle des porteurs de bandes, mais aussi d’une extinction plus ´elev´ee en infrarouge moyen. Ce fait laisse d´ej`a supposer que la saturation des bandes aromatiques que nous n’avons pas trouv´ee dans les galaxies ´etudi´ees ne se produit pas non plus significativement mˆeme dans les galaxies d’activit´es stellaires les plus extrˆemes.

Pour r´ecapituler, les relations suivantes peuvent ˆetre employ´ees (avec une pr´ecision meilleure qu’un facteur deux, mais en faisant l’hypoth`ese d’abondances solaires et d’une fonction de masse initiale de Salpeter entre 0.1 et 100 M):

−1 −9 44 −1 −2 48 SFR (M.an )=2.410 L5−8.5 µm (Lbol)pour10≤ ΣLy c (s .pc ) ≤ 5. 10

−9 44 −1 −2 45 =6.510 L12−18 µm (Lbol)pour10≤ ΣLy c (s .pc ) ≤ 3. 10

−8 46 −1 −2 48 =4.310 L15 ct (Lbol)pour3. 10 ≤ ΣLy c (s .pc ) ≤ 5. 10 , o`uΣLy c d´esigne la densit´e de flux de photons ionisants et o`u L15 ct est mesur´ee entre 14.81 et 15.19 µm. V. Formation d’´etoiles et rayonnement de la poussi`ere 141

Figure V.12: Prolongement de la relation entre flux normalis´es par la taille en Hα et `a15µm (en haut) ou 7 µm (en bas), des disques aux r´egions circumnucl´eaires. Les disques de galaxies sont repr´esent´es par des points noirs et les brillances de surface des r´egions centrales en Hα (non corrig´ees de l’extinction) par des cercles. Les brillances en Brγ, l’une non corrig´ee et l’autre corrig´ee de l’extinction, multipli´ees par (Hα/Brγ)intr = 103.8, sont marqu´ees par des croix. Les triangles repr´esentent les trois galaxies pour lesquelles je dispose seulement de flux Brγ.Les lignes continues sont les ajustements aux mesures des disques uniquement, et les lignes en tirets montrent la relation lin´eaire. 142

Figure V.13: Comparaison de la relation entre F15 ct et le flux de photons ionisants pour des r´egions des galaxies starbursts, repr´esent´ees par des signes ×,pourlesr´egions circumnucl´eaires des galaxies spirales normales (cercles : galaxies avec spectre ; carr´es : galaxies sans spectre ; triangles : galaxies sans spectre ni flux Hα ;´etoiles : intervalle autoris´e par les limites sur l’extinction dans NGC 5194), et pour les disques de galaxies. La ligne montre l’ajustement lin´eaire r´ealis´esurlesr´egions de galaxies starbursts uniquement. Pour les disques, j’ai utilis´ele rapport F15 ct/F12−18 mesur´e dans le spectre du disque de NGC 5236 (c’est le spectre de meilleure qualit´e dont je dispose). Les croix correspondent `alalimiteinf´erieure du fond zodiacal, et les points noirs `a la limite sup´erieure. Si l’on refait les mesures en utilisant les autres spectres de disques, les r´esultats sont qualitativement similaires. V. Formation d’´etoiles et rayonnement de la poussi`ere 143

Figure V.14: Comparaison de la relation entre F7 et le flux de photons ionisants pour des r´egions des galaxies starbursts, repr´esent´ees par des ´etoiles, et pour les r´egions circumnucl´eaires et les disques des galaxies spirales normales (mˆemes symboles qu’en Fig. V.12). 144

Figure V.15: Histogrammes des taux de formation d’´etoiles (en haut : totaux ; au mi- lieu : circumnucl´eaires ; en bas : dans les disques seulement), obtenus pour les galaxies de l’´echantillon en appliquant la calibration en termes de fluxa7 ` µmd´etermin´ee dans les disques. 145

Chapitre VI Activit´edeformationd’´etoiles au centre des galaxies barr´ees

VI.1 Publication

The impact of bars on the mid-infrared dust emission of spiral galaxies : global and circumnuclear properties H. Roussel, M. Sauvage, L. Vigroux, A. Bosma, C. Bonoli, P. Gallais, T. Hawarden, S. Madden & P. Mazzei accept´e pour publication dans Astronomy & Astrophysics (2001)

Je reprends ici les conclusions :

Nous avons ´etudi´e l’activit´e en infrarouge moyen induite par les barres dans un ´echantillon de 69 galaxies spirales proches, appartenant `a un large intervalle de luminosit´es infrarouges au-dessous de la classe des “galaxies lumineuses en infrarouge”. Nous avons ´etabli les faits suivants :

–L’´emission en infrarouge moyen des galaxies normales de notre ´echantillon consiste essentielle- ment en un continuum thermique de tr`es petits grains au-del`ade10µm et la famille des bandes aromatiques d´etect´ees dans des environnements tr`es divers. Ce sont les variations du continuum des petits grains par rapport aux bandes aromatiques qui sont responsables des changements dans le rapport F15/F7 dans les galaxies ´etudi´ees. En se r´ef´erant aux observations de r´egions Galactiques r´esolues, cela peut ˆetre reli´e`a une augmentation du facteur de remplissage des com- plexes de formation d’´etoiles par les r´egions ionis´ees, ce qui a pour cons´equence une diminution de la part du milieu atomique neutre et mol´eculaireal’´ ` emission en infrarouge moyen. – Il existe une dichotomie entre entre les disques spiraux, dont la couleur F15/F7 totale est proche de 1 et peu dispers´ee, et les r´egions circumnucl´eaires, o`u F15/F7 varie des valeurs des disques jusqu’`a 4. Nous n’avons pas trouv´e d’indication que la destruction des porteurs des bandes aromatiques se produise `al’´echelle des r´egions circumnucl´eaires. Il serait souhaitable d’analyser des donn´ees spectrales infrarouges pour le confirmer. – Nous confirmons que les spirales barr´ees se diff´erencient des spirales non barr´ees au sens o`u elles peuvent atteindre des rapports F15/F7 plus grands. Cet effet n’est visible que dans les types morphologiques pr´ecoces, en accord avec les ´etudes pr´ec´edentes reposant sur IRAS (Hawarden et al. 1986 ; Huang et al. 1996). Nous montrons sans ambigu¨ıt´e que cet exc`es de couleur provient 146 de r´egions circumnucl´eaires qui parfois dominent compl`etement l’´emission en infrarouge moyen, bien qu’elles soient de petite taille (leur diam`etre varie de 2 a 26% du diam`etre optique, sans d´ependance affirm´ee sur le type de Hubble). Les galaxiesaexc` ` es de couleur global sont toutes domin´ees par leurs r´egions centrales. Nos r´esultats confirment les pr´edictions des mod`eles hy- drodynamiques (Athanassoula 1992 ; Friedli & Benz 1993), selon lesquelles une perturbation barr´ee, par le couple gravitationnel qu’elle exerce et des chocs, produit un transfert de masse vers le centre. Nous observons la cons´equence de ces flots de gaz, par la formation d’´etoiles intense qu’ils alimentent. – Il faut insister sur le fait qu’une fraction seulement des galaxies barr´ees pr´ecoces se distinguent des galaxies non barr´eesdansleurspropri´et´es infrarouges. Cette faible activit´edebeaucoupde galaxies barr´ees peut s’expliquer par plusieurs param`etres, que nous ne pouvons pas explorer avec les pr´esentes donn´ees. Voici des explications plausibles : les barres ´evoluent sur des ´echelles de temps plus longues que les sursauts de formation d’´etoiles (Martinet & Friedli 1997), et le taux d’accr´etion de gaz par une barre est faible ; le transfert de masse vers le centre est r´egul´e par la profondeur du puits de potentiel, l’intensit´edeschocs`a l’int´erieur de la barre, l’efficacit´e de formation d’´etoiles sur la trajectoire du gaz avant qu’il n’atteigne les r´egions centrales (Martin & Friedli 1997), etc.

Malgr´e que la pr´esence d’une barre soit un moyen efficace de d´eclencher des sursauts cir- cumnucl´eaires de formation d’´etoiles, les m´ecanismes d’´emission de la poussi`ere dans les r´egions centrales sont identiques dans les galaxies barr´ees et non barr´ees. Nous avons ´etudi´elespro- pri´et´es de ces r´egions centrales au degr´eded´etail permis par notre r´esolution angulaire, et nous avons trouv´e que plusieurs propri´et´es physiques influencent la couleur F15/F7. – Le contenu estim´e en gaz mol´eculaire des r´egions centrales : comme attendu, la brillance `a 7 µm tend a augmenter en mˆeme temps que la densit´e de gaz moyenne, refl´etant l’association physique de la poussi`ereaveclegazmol´eculaire. La couleur F15/F7 est ´egalement corr´el´ee `aladensit´e de gaz moyenne. Comme une plus grande densit´e de gaz autorise une efficacit´e de formation d’´etoiles accrue, selon la loi de Schmidt et les crit`eres de stabilit´e (Kennicutt 1998), cela appuie une interpr´etation de la couleur en infrarouge moyen en termes d’intensit´e de la formation d’´etoiles. Quelques galaxies, aux rapports F15/F7 les plus ´elev´es, ne suivent pas le comportement g´en´eral. On peut penser `aun´etat transitoire du sursaut de formation d’´etoiles, durant lequel une grande partie du gaz aurait ´et´econsomm´ee ou dispers´ee. Des donn´ees mol´eculaires de meilleure qualit´e sont requises pour examiner cet effet. –L’ˆage des populations stellaires qui chauffent la poussi`ere : la poussi`ere est sensible `ala formation d’´etoiles sur des p´eriodes relativement longues, ce qui est attendu du fait qu’elle peut ˆetre chauff´ee par des photons optiques et en ultraviolet proche. Cependant, l’intensit´e du continuum des tr`es petits grains est plus sensible que les bandes aromatiques `aladensit´e d’´energie et la duret´e du rayonnement, et nous avons obtenu une indication que les exc`es de couleur F15/F7 sont li´es `a l’ˆage pond´er´e des populations stellaires ´emettant en ultraviolet. Ainsi, les couleurs d´ependent de l’histoire de formation d’´etoiles, et non seulement de l’intensit´ed’un sursaut contemporain.

Astronomy Astrophysics manuscript no

will b e inserted by hand later

The impact of bars on the midinfrared dust emission of spiral

galaxies global and circumnuclearprop erties



H Roussel MSauvage L Vigroux ABosma C Bonoli

 

P Gallais THawarden S Madden and PMazzei

DAPNIAService dAstrophysique CEASaclay GifsurYvette cedex France

Observatoire de Marseille Place Le Verrier Marseille cedex France

Osservatorio Astronomico di Padova Vicolo dellOsservatorio Padova Italy



Joint Astronomy Center N Aohoku Place Hilo Hawaii USA

Received January Accepted March

Abstract We study the midinfrared prop erties of a sample of nearby spiral galaxies selected to avoid Seyfert

activity contributing a signicant fraction of the central energetics or strong tidal interaction and to have normal

infrared luminositi es These observations were obtained with ISOCAM whichprovides an angular resolution of the

order of halfp ower diameter of the p oint spread function and lowresolution sp ectroimaging information

Between and m we mainly observetwo dust phases aromatic infrared bands and very small grains b oth out

of thermal equilibriu m On this sample we show that the global F F colors of galaxies are very uniform the only

 

increase b eing found in earlytyp e strongly barred galaxies consistent with previous IRAS studies The F F

 

excesses are unambiguousl y due to galactic central regions where barinduced starbursts o ccur However the

existence of strongly barred earlytyp e galaxies with normal circumnuclear colors indicates that the relationship

between a distortion of the gravitational p otential and a central starburst is not straightforward

As the physical pro cesses at work in central regions are in principle identical in barred and unbarred galaxies

and since this is where the midinfrared activity is mainly lo cated weinvestigate the midinfrared circumnuclear

prop erties of all the galaxies in our sample Weshowhow surface brightnesses and colors are related to b oth the

available molecular gas content and the mean age of stellar p opulations contributing to dust heating Therefore

the star formation history in galactic central regions can b e constrained by their p osition in a colorsurface

brightness midinfrared diagram

Key words galaxies spiral galaxies barred galaxies ISM stars formation infrared ISM continuum

infrared ISM lines and bands

Intro duction tectable Yet conclusions derived from such studies ap

p ear to contradict each other partly b ecause the dierent

As the high frequency of bars in galaxies b ecomes more

selection criteria result in samples with a more or less pro

evidenteg Eskridge et al and as new techniques

nounced bias toward starburst ob jects For instance in the

emerge to b oth observationally quantify their strength

IRbright sample analyzed byHawarden et al an

Seigar James Buta Blo ck and numer

imp ortant fraction of SB and SAB galaxies resp ectively

ically simulate them their eects on their host galaxies

strongly barred and weakly barred spirals in the classica

are of ma jor interest and in particular it is worth check

tion of de Vaucouleurs et al shows a m emission

ing whether they are indeed very ecient systems to drive

excess with resp ect to and m absent in the SA

nuclear starbursts in spiral galaxies

subsample nonbarred spirals which can b e accounted

Numerous studies have dealt with the resp ective star

for by a highly increased contribution of GalacticlikeHII

formation prop erties of barred and nonbarred spirals

regions to the total emission On the other hand Isob e

mostly in the infrared since this is the wav elength regime

Feigelson using a volumelimited sample and

where starbursts are exp ected to b e most easily de

p erforming a survival analysis to takeinto account the

frequent IRAS nondetections found that the farIR to

Send oprint requests to H Roussel email hrousselceafr

blue ux ratio F F is rather indep endent of the bar

Based on observations with ISO an ESA pro ject with in

FIR B

class The contradiction is marginal since F F do es struments funded by ESA Memb er States esp ecially the PI

FIR B

countries France Germany the Netherlands and the United

not give a direct estimation of the star formation activity Kingdom and with the participation of ISAS and NASA

H Roussel et al Impact of bars on MIR dust emission of spirals

ing in view of the very dieren esp ecially when dealing with quiescent normal galaxies t timescales of bar evolu

the blue light originates partly from young stars and as tion
Gyr and star formation in kp cscale regions

Isob e Feigelson emphasize F F dep ends on
yr Indeed Martinet Friedli using

FIR B

the amount and spatial distribution of dust with resp ect carefully selected latetyp e galaxies found no such cor

to stars The relationship b etween the m excess quan relation the bar strength b eing quantied either byits

tied by F F and F F in a galaxy sample with depro jected axis ratio or its depro jected length relative

FIR B

go o d quality data is indeed highly disp ersed Huang et to the disk diameter The fact that only a fraction of

al investigated the m excess as a function of strongly barred galaxies exhibit star formation excess as

IR brightness and reconciled the two previous analyzes evidenced by their IRAS colors is explained by these au

a signicant excess can o ccur only if F F is larger thors with numerical simulations of bar evolution includ

FIR B

than a threshold value of Therefore a statistical ing gas physics They show that a strong starburst o ccurs

eect of bars on star formation can b e demonstrated only shortly after bar formation and quickly fades away in typ

in suitably selected samples Huang et al also em ically less than Gyr meanwhile the strength and other

phasized that the dierence b etween barred and unbarred prop erties of the bar evolve but the bar remains strong

spirals concerns only early typ es Sa to Sb c if it was initially strong The existence of strongly barred

Studies of the infrared excess in barred galaxies mostly galaxies in a quiescent state is thus to b e exp ected pre

rest on the integrated IRAS measurements whichdo sumably b ecause the available gas supply has b een con

not allow the determination of the nature and lo cation sumed in previous bursts

of regions resp onsible for this excess However dynam This pap er is aimed at characterizing the midinfrared

ical mo dels and observations at other wavelengths give excess in barred galaxies with the p ossibility to carry

evidence that the infrared activity should b e concen out a detailed and systematic spatial analysis due to

trated in circumnuclear regions see for instance the study the go o d angular resolution of ISOCAM the halfp ower



of NGC by Sheth et al In addition high b eam diameter is less than at m and hence to

resolution groundbased observations near mofgalaxy lo cate unambiguously sites of enhanced infrared activity

centers Devereux Telesco et al haveshown Although dust is a more indirect tracer of young stars

that the dust emission is more concentrated in barred than farultraviolet ionizing radiation or optical recombi

galaxies nation lines the infrared emission suers relatively minor

Theoretically bars are known to b e resp onsible for extinction eects which are very dicult to correct and

largescale redistribution of gas through galactic disks In hamp er shorter wavelength studies In a companion pa

a strong barred p erturbation of the gravitational p oten p er Roussel et al a hereafter Pap er I I wehave

tial sho cks develop along the rotationleading side of the shown that in galactic disks midinfrared emission is a

bar and are asso ciated with strong shear as shown by reliable star formation indicator Here we concentrate on

Athanassoula and references therein also Friedli central regions of galaxies where the dust heating regime

Benz They induce an increase of gas density is markedly dierent from that in disks

which is traced by the thin dust lanes widely observed in For this purp ose weha ve analyzed a sample of

bars pro ducing a contrasting absorption of optical light nearby spiral galaxies imaged at and m with the

Prendergast unpublished Huntley et al Due camera ISOCAM on b oard ISO describ ed by Cesarsky

to these sho cks gas loses angular momentum and ows et al c Wehave also obtained lowresolution sp ec

towards the circumnuclear region This picture is con troscopic information for a few galaxies enabling us to

rmed by direct observations of inward velo city gradients identify and separate the various dust comp onents emit

across bars in ionized gas lines CO and HI eg Lindblad ting b etween and m m images and F F ux

et al Reynaud Downes Mundell Shone density ratios of selected regions together with optical im

Regan et al derive a gas accretion rate of ages are presented in Roussel et al b hereafter the

M yr into the circumnuclear ring of NGC Atlas For a description of data reduction and analysis

Statistical evidence is also found for higher gas con and a summary of morphological prop erties of the sample

centrations in the center of barred galaxies Sakamoto the reader is also referred to the Atlas

et al who however observed only SABs except

NGC and for more frequent circumnuclear star

The galaxy sample

bursts in barred galaxies as rep orted by Heckman

Hawarden et al Arsenault who more ex The sample is intended to b e representative of normal

actly found more probable starbursts in galaxies with quiescent spirals and contains galaxies of mo derate in

b oth bar and inner ring supp osed to b e a signature of frared luminosityItcovers three guaranteed time pro

one or two inner Lindblad resonances Huang et al grams of ISOCAM The rst one Cambarre consists of

Martinet Friedli and Bonatto et al nearby barred galaxies the second one Camspirofa

Aguerri has moreover rep orted that the few largesize spirals of sp ecial interest NGC

global star formation intensity of isolated spirals mostly and and another subsample is

of late typ es is correlated with bar strength as quantied drawn from the Virgo cluster sample of Boselli et al

by means of its pro jected axial ratio which is surpris Virgo program containing relatively fainter and smaller

H Roussel et al Impact of bars on MIR dust emission of spirals

mean value of For that set of references a Wilcoxon galaxies b oth barred and unbarred This sample was sup

MannWhitney WMW test indicates that the probabil plemented by comparable spirals in the ISOCAM public

ity for the two p opulations to have the same F F archive from the programs Sf glx Dale et al and

FIR B

distribution is ab out We note that the IRAS m Irgal PI T Onaka All of the observations were reduced

uxes often disagree with our and m uxes although in the same way to form a homogeneous sample The

the bandpasses overlap Thus when we use IRAS data we nal set comprises spiral galaxies at distances b etween

take them from the references we consider the most reli and Mp c Wehave divided them into three main cat

able ie which provide the b est matchbetween mand egories according to morphological classes in the RC de

our m ux densities In that case log F F Vaucouleurs et al SBs accounting for ab out half

FIR B

falls in the interval
with a mean value of the sample with galaxies SABs galaxies and SAs

and the WMW test gives a probability of ab out galaxies The latter two classes are merged to form

Hence our sample is not dierent from optically complete the control sample to compare with SB galaxies This sam

samples regarding the fraction of the energy radiated in ple although not statistically complete has b een selected

the infrared according to the following requirements

Table lists some general characteristics of the galax All ob jects are relatively nearby which ensures go o d

ies The morphological classication adopted is that of the spatial resolution with a or pixel size the ex

RC de Vaucouleurs et al Although it is based on tension of the central concentration is typically

blue images whichmay not b e as appropriate as near pixels in diameter At the distances of the sample

infrared images for detecting bars many more galaxies a pixel corresp onds to linear sizes b etween and

are classied as barred in this catalog than for instance in p c Virgo galaxies less extended and all imaged

e Wehave found only two galax Sandage Bedk with pixels in order to increase the signal to noise

ies classied as SA in the RC and p ossessing a bar as ratio are resolved but with less detail

describ ed in the following A drawback of using the SB The sample was selected to avoid nonstellar activity

and SAB classes of the RC is that they do not consti as well as strong signs of tidal interaction with a few

tute a measure of the bar dynamical strength The bar exceptions in the Cambarre and Virgo subsamples de

strength is however dicult to quantify and reliable mea tailed in the following

sures such as those of Buta Blo ck are scarce Galaxies included in the rst two programs are mo d

In the following we will refer to bar lengths normalized erately inclined on the line of sighti This

by the disk diameter b ecause longer bars are able to col was not a requirement for the other programs so that

lect gas from inside a larger area and havelow axis ratios one third of the Virgo galaxies and one third of the

which are among the unsatisfactory quantities used to supplementary galaxies are inclined by more than

estimate bar strengths bar lengths are in addition rela Thenumb er of SASAB galaxies is comparable to that

tively easy to measure of SBs and b oth groups span the whole de Vaucouleurs

spiral sequence from typ es Sa to Sdm

The two subsamples of spirals found in the eld or

Both barred and unbarred galaxies cover a large

lo ose groups and Virgo galaxies have b een separated b e

range of farinfrared luminosities b etween and

cause they dier b oth in their asp ect in the infrared Virgo

L but none would b e classied as an in

bol

memb ers are fainter and less extended and in their en

frared luminous galaxy except NGC whichisat

vironment Although Virgo is not a very rich cluster

the lower b oundary of this class dened by

the interaction of central galaxies with the intracluster

L L Sanders Mirab el

FIR
bol

gas and with their neighbours is likely to cause either

a depletion or an enhancement of star formation activ Another prop erty whichwas not a selection criterion

ity in the outer parts of disks and also to have global is that absolute blue magnitudes are equal to or greater

dynamical consequences An extreme case is the galaxy than the typical magnitude of the Schechter luminosity



NGC VCC whose very p erturb ed morpho function in the eld M
more exactly they range

B

logical app earence was successfully mo delled by Combes between and

et al as the result of a collision with NGC Despite the incompleteness of the sample wehave

Several Virgo memb ers have truncated HI disks due to checked that it is very similar to the magnitudelimited

the interaction with the cluster hot gas Cayatte et al CfA galaxy sample Thuan Sauvage from the

a very clear example is NGC VCC p oint of view of its infrared brightness normalized by

which on optical photographs shows the juxtap osition of blue starlight For CfA spiral galaxies detected in all

a bright and patchy inner disk structured by star forma IRAS bands and with blue magnitudes in the RC

tion sites and dust lanes and a low surface brightness and log F F falls in the interval
with a

FIR B

very smo oth outer disk with faint spiral arms Severely mean value of Using the same IRAS references as

H ie by order of prefer Istripp ed galaxies can indeed b e recognized in the op those in ThuanSauvage

ence Thuan Sauvage Rice et al Soifer et tical as anemic dened byvan den Bergh as an in

al and Moshir et al galaxies in our sample termediate and parallel sequence b etween lenticulars and

have log F F in the interval
with a spirals due to the suppression of star formation where

FIR B

H Roussel et al Impact of bars on MIR dust emission of spirals

a reference zo diacal sp ectrum to the average sp ectrum of the gas densityistoolow Table also indicates whether

the faintest pixels excluding the sp ectral regions where signatures of nuclear activity or tidal interaction exist

emission features app ear The upp er limit to the zo diacal

In addition to these some galaxies deserve sp ecial com

foreground is set by osetting the tted sp ectrum within

ments see the Atlas for more details and should b e con

the disp ersion range with the additional constraint that

sidered cautiously in the interpretation of the data set

the corrected disk sp ectrum remains p ositive the lower

NGC and are strongly asymmetric and

limit is symmetric to the upp er limit with resp ect to the

t in the category of magellanic barred spirals

t This makes little dierence for the nuclear sp ectra but

NGC and have an amorphous structure and

it do es for the disk sp ectra although it do es not aect the

highly centrally concentrated interstellar tracers Their

sp ectral shap e Note that due to the conguration of the

morphology is suggestive of merger results Their star

instrument two dierent lters are used for the short and

formation activitymay therefore not b e a consequence

long wavelength parts of the sp ectra and that a small

of the bar since the latter was likely pro duced at the

oset can result at the junction of these lters around

same time by the same cause ie the merger

m

The midinfrared maps generally showanintense cir

cumnuclear source Decomp osing surface brightness pro

Observations and photometric results

les into a central condensation and a disk see details

All galaxies were observed with two broadband lters

in the Atlas we dene a radius for this circumnuclear re

LW m and LW m that we shall here

gion R Total uxes and uxes inside R are listed

CNR CNR

after designate by their central wavelength resp ectively

in Table with the background level for each broadband

and m This was exp ected to provide F F col

lter Explanations ab out the metho d employed for pho

ors directly linked with star formation intensity since the

tometry and the estimation and meaning of errors can

LW lter covers the emission from a family of bands see

b e found in the Atlas The dominant uncertainty arises

Sect which are ubiquitous in the interstellar medium

from memory eects for relatively bright galaxies and

and LW was supp osed to cover mainly a thermal con

from other sources of error essentially the readout and

tinuum observed to rise faster than the emission bands in

photon noise for faint galaxies esp ecially at m For

starforming regions for instance from the IRAS F F

galaxies drawn from the Sfglx pro ject the number of ex

ratio Helou however we will see that the picture

p osures p er sky p osition is very small and do es

is more complicated Maps cov ering the whole infrared

not allow a prop er estimate of memory eects their pho

emitting disk were constructed in raster mo de In all cases

tometric errors are thus esp ecially illdetermined Typical

the eld of view is large enough to obtain a reliable de

errors are  at m and at m Note that ux

termination of the background level except for NGC

density calibration uncertainties which are of the order of

and The pixel size is either or dep ending on

to are not included However this is a systematic

the galaxy size The halfp owerhalfmaximum diameters

eect hence not aecting relative uxes

of the p oint spread function are resp ectively

at mwitha pixel size at m with a

Nature of the midinfrared emitting sp ecies

pixel size at m with a pixel size and

The sp ectra shown in Fig are strikingly similar to one

at m with a pixel size The data reduc

another They contain some features also seen in sp ectra

tion is describ ed in the Atlas

of reection nebulae atomic and molecular envelop es of

Since the emission from various dust sp ecies and

HII regions atmospheres of Crichevolved stars as well as

atomic lines is mixed in the broadband lters see Sect

the diuse interstellar medium We can thus safely assume

it is essential to complement our maps with sp ectro

that the results obtained on these resolved Galactic ob

imaging data These allow an estimate of the relativeim

jects can b e readily extrap olated to the emission of galax

p ortance of all sp ecies as a function of the lo cation in

ies where individual sources are no longer resolved

side a galaxyWehavethus obtained sp ectra b etween

The emission b etween and m is dominated by

of and m of the inner disks
or

the socalled unidentied infrared bands UIBs at

ve bright galaxies NGC and

and m Our sp ectra also displayweak

Fig Sp ectra averaged over a few central pixels cover

features whichhave previously b een detected as broad fea

ing approximately the extent of the circumnuclear region

tures in SWS sp ectra of starburst ob jects Sturm et al

left column are compared with sp ectra averaged over

at eg and m

the inner disk excluding the central part and a p ossible

A m feature can tentatively b e identied as an ArII

ghost image middle column The right column shows

the observed sp ectrum of the faintest pixels consisting of

We also detect a weak and unknown emission feature b e

the zo diacal sp ectrum contaminated by emission features

tween and m which seems brighter relatively to UIBs

from the target galaxy b ecause the eld of view never

in disks than in central regions However the very p o or signal

extends b eyond the galactic disk For this reason wecan

to noise ratio of disk sp ectra do es not allow us to b e conclusive

not measure exactly the level of the zo diacal foreground

It cannot b e an artefact due to the change of lter since that

change o ccurs after the feature is observed It is to o narrowto to remove Instead as explained in the Atlas we rst t

H Roussel et al Impact of bars on MIR dust emission of spirals

sion from VSGs and UIB carriers can b e clearly separated line m or an H rotational line m but our

around M and in the reection nebula NGC the sp ectral resolution prevents a more de

VSG continuum strongly p eaks in a layer closer to the ex nite identication We note however that the ArII line

citation sources than the UIBs inside the ionized region has b een identied in the highresolution SWS sp ectra of

for M Cesarsky et al a b Therefore the starburst galaxies Sturm et al

F F ux ratio decreases with increasing distance from

It was originally prop osed by Duley Williams

the exciting stars of an HII region

that UIBs are due to organic functional groups on car

In the sp ectra of all ve galaxies Fig the intensity

b onaceous grains Leger Puget instead favoured

ratios of UIBs are remarkably stable which is a common

vibration mo des of CC and CH b onds only in large

prop ertyofavariety of astronomical sources Cohen et

p olycyclic aromatic molecules not in thermal equilibrium

al Uchida et al The only highly varying fea

with the lo cal radiation eld the socalled PAH mo del

ture is the VSG continuum that is b est seen longward of

The constancy of the sp ectral energy distribution of UIBs

m It has various amplitudes and sp ectral slop es in

regardless of the radiation eld Sellgren Uchida

galactic nuclei It remains very mo dest compared to that

et al implies an impulsive heating mechanism

in starburst galaxies Tran Sturm et al and

where up on absorption of a single UV photon the car

is hardly presentinaveraged disks In Pap er I I weshow

riers undergo a very rapid and large temp erature increase

that the integrated midinfrared luminosity of normal spi

and then radiatively co ol b efore the next absorption

ral disks is dominated by the contribution from photo dis

Alternative candidates for the UIB carriers are various

so ciation regions where the UIB emission is maximum

hydrogenated and oxygenated carb on grains amorphous

From a comparison with H luminosities we showthat

but partially ordered at the smallest scale Borghesi et

this predominance of the photo disso ciation region emis

al Sakata et al Pap oular et al much

sion results in the fact that when integrated over the disk

similar to the idea of Duley Williams Recent

the UIB emission is a go o d tracer of massiveyoung stars

work by Boulanger et al b indicates that UIBs are

Finally as alluded to earlier a numb er of nestructure

not due to molecules suchasPAHs but more likely to

lines can b e present in the midinfrared sp ectral range

aggregates of several hundred atoms

although their contribution to the broadband ux is al

In the interstellar medium surrounding the OB asso ci

ways negligible in spirals In normal galaxies the most

ation Trap ezium Ro che et al the Orion bar Giard

prominent is the NeII line at m whichatthe

et al and M Cesarsky et al a Tran

sp ectral resolution of ISOCAM is blended with the UIB

these features are detected in the HII region and the molec

at m No lines from high excitation ions suchas

ular cloud front provided pro jection eects are minor

NeIII at m are convincingly detected and the

but the emission p eaks at the photo disso ciation interface

NeII line at misweak since the intensityofthe

see also Bro oks et al UIB carriers are likely de

blend with the UIB at m relative to the isolated

stroyed in HII region cores although the estimation of the

UIB at m is rather stable in dierent excitation

critical radiation eld necessary to obtain a signicant re

conditions Some variation however exists To compare the

duction in UIB carrier abundance still remains to b e done

strength of the NeII line in our galaxies to that observed

compare eg Boulanger et al a Contursi et

byForsterSchreib er et al in the starburst galax

al

ies M NGC and NGC wehave measured in a

While the m ux in spiral galaxies essentially con

similar way the ux of the blend F ab ove the pseudo

sists of the UIB emission the mltercovers the emis

continuum drawn as a straight line b etween and

sion from mainly two dust sp ecies the hot tail of a contin

m and the ux of the mUIB F with its

uum attributed to very small grains VSGs of the order

resp ectiv e continuum level dened in the same waybe

of nm in size and most often impulsively heated

tween and m We nd that the energy ra

like UIB carriers Desert et al and also UIBs The

tio F F of circumnuclear regions decreases from

red wing of the m band contributes little but the

in NGC to in NGC and in

band at m and the emission plateau that connects

NGC and in NGC in the averaged inner

it to the m band can b e imp ortant the smaller UIB

disks of NGC and where it is still mea

features listed ab ove also contribute although to a lesser

surable it takes the approximate values and

extent When spatial resolution is high enough the emis

These gures are muchlower than those observed in cores

b e emitted by silicates The identicati on with ionized PAHs

of starburst galaxies byForsterSchreib er et al

see eg Allamandola et al would b e inconsistent with

where it can reach and argue for a generally small

the fact that the ux ratio of this feature to classical UIBs

contribution of NeII to the sp ectra Adopting as the in

seems higher in regions of low radiation density and excitation

trinsic F F UIB energy ratio the minimum value of

than in central regions It is also unlikely that it corresp onds

F F that we measure in our sp ectra ie we

to the H rotational line at m since this is characteristic

obtain a maximum NeII equivalent width of min

of warm and excited molecular clouds in starburst nuclei eg

the nucleus of NGC As for the UIBs their equivalent

Sp o on et al Finallywe mention that it also matches in

widths in disks and central regions range resp ectively b e

wavelength a feature from the CH functional group at m

Duley Williams tween EW m and EW m

H Roussel et al Impact of bars on MIR dust emission of spirals

Fig Sp ectra of central regions left and the inner disk middle The upp er and lower limits are determined from limits on

the zo diacal sp ectrum shown with dotted lines right adjusted using the average sp ectrum of the faintest pixels also shown

with its disp ersion The ux unit for all sp ectra is mJy arcsec

H Roussel et al Impact of bars on MIR dust emission of spirals

these numb ers do not takeinto account broad UIB wings

that o ccur if the bands are describ ed byLorentzians Our

estimates give a minimum value for F F of in

NeI I

NGC that we can compare with the results of Sturm

et al in the starburst galaxies M and NGC

from their ISOSWS sp ectra with a high sp ectral resolution

versus for ISOCAM They obtain

values of and The contribution from the NeII

line to our sp ectra is thus conrmed to b e negligible with

resp ect to starburst galaxies

The dep endence of F F on spiral and bar



typ es and comparison with IRAS results

In their infrared analysis Huang et al p ointed out

that bars are able to signicantly enhance the total star

formation only in earlytyp e galaxies mixing all typ es

between Sa and Sb c It is also known that bars do

not share the same prop erties all through the Hubble se

quence among early typ es or more exactly in spirals with

large bulges since the relationship b etween Hubble typ e

and bulge to disk ratio is far from direct eg Sandage

Bedke Seigar James they tend to b e longer

Athanassoula Martinet Martin and their

amplitude with resp ect to that of the underlying axisym

metric p otential tends to b e higher For instance Seigar

James using K band photometry to trace the

stellar mass nd that galaxies with the strongest bars

have bulge to disk mass ratios b etween and For

larger bulges their numb er of galaxies is to o lowtoderive

any meaningful bar strength distribution Earlytyp e bars

host little star formation except near their ends and at

their center whereas latetyp e galaxies generally harb or

HII regions all along the bar Garc aBarreto et al

Fig a Integrated midinfrared color F F as a function

which suggests that their sho cks are not as strong as in

of morphological typ e for unbarred or weakly barred galaxies

early typ es Tubbs Inner Lindblad resonances b e

represented resp ectively by op en circles and crossed circles

tween the gas and the densitywave which app ear when

Virgo galaxies are identied by their VCC number see Table

there is sucient central mass concentration and when

and others by their NGC number b Same as a for strongly

the bar rotates more slowly than where

max

barred galaxies

is the gas circular rotation frequency and the epicyclic

frequency and which presence induces straight and o

infrared color is remarkably constant around a valueof

set sho cks along the bar Athanassoula are also

ranging from to This is rather typical of the color

typically exp ected in earlytyp e galaxies These structural

of the surface of molecular clouds exp osed to radiation

dierences have consequences on the eciency of bars to

elds ranging from that observed in the solar neighbor

drive massiveinward gas ows

ho o d to that found in the vicinity of starforming regions

We show in Fig a the distribution of F F accord

F F colors observed toward HII regions are typically

ing to morphological typ e as given in the RC for the

of the order of while those of photo disso ciation re

control subsample including only SA and SAB galaxies

gions range b etween and the HII region values Tran

For this p opulation excepting NGC the mid

The fact that F F remains of the order of

This galaxy shows a p eculiar structure with a central lens

in most galaxies it also shows generally little variation

like b o dy or fat oval of mo derate length D D sur

bar

from pixel to pixel in disks indicates that at our angular

rounded by an external pseudoring probably asso ciated with a

resolution emission from HII regions and their immediate

Lindblad resonance NGC is thus a genuine weakly barred

surroundings is diluted by the larger neighb oring interstel

galaxy and not a SB However such a dynamical structure

lar medium at a mean distance of Mp c represent

is still ecient to driveinward mass transfer We also p oint

p c In fact in the Atlas weshow that even in giant

to the very strong concentration of its midinfrared emission

starforming complexes that can b e identied in the maps

see Table a prop erty common to earlytyp e barred spirals

Fig F F rarely exceeds

H Roussel et al Impact of bars on MIR dust emission of spirals

The case of strongly barred spirals is more complex

Fig b whereas many of them share the same integrated

colors as their unbarred counterparts an imp ortant frac

tion shows a color excess the maximum color b eing ab ove

instead of for SABs Furthermore such an ex

cess o ccurs only among the earliest morphological typ es

from SBa to SBb Note that in bulges the envelop es

of KM stars can contribute an imp ortant fraction of the

midinfrared emission However this would b e negligible

at m and mostly aect the m band correcting for

such an eect would only reinforce the observed trend

We also qualify that observation by noting that twogalax

ies NGC and NGC havelikely exp erienced a

merger gas may therefore have sunk to the center as a

result of the violent energy dissipation in the merger and

not simply under the inuence of the bar which actually

mayhave b een formed during the interaction Dismissing

these two ob jects however do es not change the fact that

Fig F F Comparison of midinfrared colors from ISO



the color distribution of the strongly barred galaxies shows

F F F and IRAS always contains the VSG emission


 

F see Sect whereas at low temp eratures is dominated by

m excesses that are absentfromthatofweakly barred

F F below a thresh UIBs which explains the constancy of



or unbarred spirals

F F old of The same convention as in Fig applies




One can wonder whether cluster galaxies intro duce a

for the representation of SA SAB and SB classes Wehave

bias in our sample b ecause a numb er of them are p er

indicated the names of the Sy galaxy NGC VCC

turb ed by their environmentandthus mayhaveanun

F F see Sect and of the two galaxies with the lowest



certain morphological typ e Ko opmann Kenney

colors note that NGC was not entirely mapp ed and that

yp e spi have shown that a signicant fraction of earlyt

its integrated color is likely a lower limit

rals in Virgo have b een misclassied due to their dearth

of star formation in the disk The degree of resolution of

a direction distinct from the ma jor axis of outer isophotes

spiral arms into star formation complexes is indeed one

and crossed by dust lanes it is furthermore classied as

of the three criteria dening the Hubble sequence but

suchbyVorontsovVelyaminov Arkhip ova and

it is not unambiguously linked to the bulge to disk ra

Corwin et al These galaxies show no global color

tio Concerning several Virgo memb ers of our sample the

excess like the rest of the SASAB subsample and likea

bulge is very small for the attributed typ e Sandage

numb er of b onade earlytyp e SB galaxies with normal

Bedke in such cases dened mostly by the disk

HI content Hence our view should not b e to o strongly dis

app earance This is of course related to the anemia phe

hecked the torted by the classication bias Wehave also c

nomenon due to gas deciency caused byinteraction with

inuence of this bias on HIdecient barred galaxies with

the intracluster medium Of our Virgo galaxies of typ es

a color excess On optical images VCC NGC

SaSb are HIdecient versus for typ es Sb c

unambiguously resembles classical earlytyp e spirals with

Sdm see Table where def has b een adopted as

a prominent bulge crossed bythick dust lanes VCC

the criterion for HI deciency This apparent segregation

NGC stands b etween HIdecientandHInormal

with morphological typ e certainly results from the ab ove

galaxies and also has an earlytyp e asp ect The case of

classication bias Thus dierentiating galaxies in Fig

VCC NGC is not that clear b ecause it is a

according to their true bulge to disk ratio would cause an

lowmass galaxy

underrepresentation of SASAB earlytyp e spirals which

make the crucial part of our comparison sample If we Our sample thus conrms and extends to the ISOCAM

had to discard completely the earlytyp e SASAB subsam bands a phenomenon that was evidenced from IRAS ob

servations byHawarden et al and Huang et al

ple the maximum allowed conclusion from Fig would b e

namely that a signicant fraction of SB galaxies

that we observe a color excess in a fraction of earlytyp e

can show an excess of m emission normalized to the

strongly barred galaxies without excluding the p ossibil

ityofsuch an excess in earlytyp e nonbarred galaxies emission at or m compared with SA and SAB

in which case another mechanism for mass transfer would galaxies The case of SAB galaxies is in fact unclear some

of them showsuch an excess according to Hawarden et

have to b e thoughtof

al but they are indistinguishable from SAs in the

However at least ve earlytyp e SASAB spirals re

analysis of Huang et al From the present ISOCAM

main which are not HIdecient and thus unlikely to suf

data it already app ears that indeed SA and SAB galaxies

fer from the ab ove bias namely VCC NGC

NGC NGC NGC and NGC Wedo share similar midinfrared prop erties

not consider NGC SAa in the R C b ecause it In order to compare more directly our results with

lo oks like a genuine barred galaxy its bulge is elongated in IRASbased results Fig shows the relationship b etween

H Roussel et al Impact of bars on MIR dust emission of spirals

F F and F F For log F F whichis prop erties of the central regions as dened in Sect and

close to the value given byHawarden et al as the in the Atlas

limit for the presence of a m excess F F shows no

systematic variation and SA SAB and SB galaxies are well

The role of central regions

mixed Ab ove that threshold SB galaxies strongly domi

nate the SAB galaxy with high colors is NGC and

As the presence of a bar is exp ected to inuence the star

the F F ratio follows the increase of F F Given the

formation in the circumnuclear region and muchlessin

nature of dust comp onents whose emission is covered by

the disk except in the zone swept by the bar weare

the mtom lters Sect this b ehavior can b e

naturally led to emphasize the relative prop erties of nuclei

explained as follows The classical interpretation for the

and disks Maps shown in the Atlas demonstrate that cen

variation of F F is that it increases with the radia

tral regions observed in the infrared are prominentand

tion eld due to the stronger contribution of VSGs to the

clearly distinct from other structures much more than on

m than to the m emission which collects mostly

optical images

UIB emission Desert et al Helou The fact

In Fig we plot the fraction of the total m

that the F F ratio remains insensitive to the variation

ux originating from the central region inside the radius

for log F F implies that in this of F F

R as a function of the global F F color Galaxies

CNR

regime VSGs provide little ux to b oth ISOCAM bands

h a central region could not b e dened on the for whic

as well Past this threshold the increase of F F signals

midinfrared brightness proles are also shown and are

that the VSG continuum has entered the m bandpass

attributed a null central fraction Galaxies are not dis

and contributes an ever increasing fraction

tributed at random in this plot but rather on a twoarm

sequence that can b e describ ed in the following way

The galaxies with a m excess F F ab ove or

high F F colors are found exclusively in systems where

dex also distinguish themselves from the rest of our

a high fraction of the ux is pro duced in the circumnuclear

sample byhaving on average larger farinfrared to blue

regions galaxies with small F F ratios are

luminosity ratios For this subsample L L spans the

FIR B

found with all kinds of nuclear contributions

range with a logarithmic mean of and disp er

The bar class app ears to play a part in the lo cation of

sion by a factor while L L of the complementary

FIR B

galaxies in this diagram although this is not clearcut all

subsample falls in the interval has a logarithmic

galaxies with high circumnuclear contribution

mean of and disp ersion by a factor However the

and large F F colors are SB galaxies apart

mexcess galaxies have farinfrared luminosities that

from NGC while SASAB galaxies are quite indis

are equivalent to those observed in the rest of the sample

tinguishable from one another and cluster in the small

Hence in these galaxies with a VSG emission excess a

nuclear contribution and low F F color cor

higher fraction of the total emission is repro cessed in the

ner of the graph There is also a clear prep onderance of

whole infrared range There is also a slight dierence al

SB galaxies in all the centrally dominated range Only two

though not statistically signicant b etween SBs with no

SASAB galaxies showvery high concentration fractions

m excess and SASAB galaxies the L L logarith

FIR B

mic means and disp ersion factors are and for SBs

with no excess and and for SAsSABs

That midinfrared color excesses o ccur only in SB

galaxies indicates that somehow a global increase of the

interstellar radiation eld intensity is linked to the pres

ence of a strong bar although this condition is clearly not

sucient The fact that many barred galaxies earlier than

SBb app ear very similar in their integrated color to their

unbarred counterparts means that no simple link exists

between the bar class the bulgetodisk ratio and the on

set of a starburst in normal spirals Several intervening

parameters can b e thought of the true strength of the

bar in dynamical terms the separation into SB and SAB

classes is sub jective and to o rough and in a recent study

Buta Blo ck show that the SB class includes a wide

range of actual bar strengths the available gas content

inside corotation the star formation eciency along bars

and in central regions the timescales for starburst acti

vation and exhaustion interaction with a companion or

Fig Relationship b etween the central ux fraction at m

with the intracluster gas Some of these eects can b e in

and the integrated F F color Galaxies with no identiable



central regions ie surface brightness proles consistentwith

vestigated in the present sample We will discuss them in

a single disk comp onent have b een placed at a null ordinate

Sect but rst we turn our attention to midinfrared

H Roussel et al Impact of bars on MIR dust emission of spirals

NGC and NGC The latter galaxy was already

discussed for NGC strong indications exist that its

bar class is incorrect see the discussion in Sect

However it is quite signicant that SB galaxies cover

b oth sequences in Fig and in particular are found all

through the sequence of varying ux concentration and

low F F color Therefore Fig shows that high global

F F colors require that the ux concentration b e high

and that the galaxy b e SB but none of these two prop er

ties is enough to predict that the global F F ratio will

b e high To understand the imp ortance of the ux concen

tration let us rst study separately the colors of central

regions and those of disks

Fig compares the F F distributions observed in

the disk and in the central regions of our galaxies when

ever the radius of the central regions R tted on

CNR

m brightness proles could not b e dened the galaxy

Fig F F colors averaged in Compared histograms of



has b een considered as a pure disk These histograms

disks and in circumnuclear regions The galaxies used are re

indicate that F F ratios of circumnuclear regions are

sp ectively those whose disk is not strongly contaminated by

higher than those of disks and this is a systematic prop

the central comp onent the excluded galaxies are NGC

ertyveried for each individual galaxy except NGC

NGC VCC NGC NGC and NGC

and whose central regions are dominated byold

and those with central regions that could b e adjusted on sur

stellar p opulations Colors of disks are fairly constant

face brightness proles otherwise the galaxy is considered to

and close to the integrated colors of SASAB galaxies

b e comp osed only of a disk The isolated galaxy with a very

F F for the disp ersion whereas cir

lowcentral color is NGC which is clearly devoid of young

stars all inside its inner ring

cumnuclear colors form a broader distribution extending

to wards high values F F

The cause for this dierence of colors can easily b e seen

in the sp ectra of Fig in all sp ectra with sucient signal

tonoise ratio the relativeintensities of the UIBs are al

most unchanged from galaxy to galaxy or from central

regions to disks On the contrary the level and sp ectral

slop e of the continuum seen longward of mishighly

variable and always stronger in the central regions than

in the disks This continuum is attributed to VSGs see

Sect and its presence in the mbandisa charac

teristic sign of intense star formation eg Laurentetal

The reason why high global F F colors require a

high ux concentration can b e directly derived from Fig

only the central regions of galaxies are able to reach high

F F colors and they havetodominatetheintegrated

emission to aect the global color Furthermore the fact

that the two color histograms overlap explains whya

Fig Fraction of total m uxes arising from the central

high ux concentration do es not necessarily imply a high

condensation as a function of morphological typ e As in Fig

F F color

galaxies with no identiabl e central regions have b een placed

Wehowever still havetoidentify the prop ertyorprop

at a null ordinate The central fraction of F uxes not shown



erties required in addition to b elonging to the SB class for

here has a very similar b ehavior with only slightly lower val

a galaxy to show a high midinfrared ux concentration

ues

Wehave seen in Fig that the morphological typ e plays

a ma jor part in the presence of high colors Fig shows

the evolution of the concentration fraction as a function It is less clear in Fig whether SASAB galaxies fol

of morphological typ e It conrms that for SB galaxies low a similar or a dierent trend partly b ecause of the

there is a denite trend for the central ux fraction to rise lackofsuch bar classes in our sample for typ es Sa and

as the morphological typ e gets earlier More preciselySB Sa and also b ecause typ es Sab and Sb may b e incorrect

galaxies with central fractions greater than are found due to the morphological classication bias aecting clus

predominantly among galaxies earlier than Sb ter galaxies as already discussed in Sect In that section

H Roussel et al Impact of bars on MIR dust emission of spirals

for more detail we obtain fractions of the total circum however we emphasized the existence of a set of ve early

typ e SASAB spirals which do not suer from morpho nuclear uxes contributed by the nuclear p oint source of

logical misclassication VCC NGC NGC less than at m and ab out at m This central

NGC NGC and NGC As apparentin source was measured inside a radius of while the ring

Fig they all havealow central ux fraction muchlower extends b etween radii and

than that observed in SB galaxies in the same range of

We can also insp ect the lowresolution sp ectra b etween

typ es This supp orts the view that the trend seen for in

and m of the central regions of NGC and

creasing concentration fraction with earlier typ e concerns

left column of Fig Indeed Genzel et al and

only SB galaxies or p eculiar ob jects likeVCC SA

Laurent et al haveshown that a strong contin

SAB galaxies having a generally low concentration factor

uum at m and small equivalent widths of the UIBs

whatever their typ e

are signatures of dust heated by an activenucleus Yet

We can summarize our ndings in this section in the

all our sp ectra are similar to that of the inner plateau

following way integrated F F colors of galaxies are

of NGC in diameter which also contains

generally of the order of However F F is often higher

aweak Seyfert nucleus but completely negligible and

in central regions Spiral galaxies with high F F colors

to that of NGC they are dominated by UIBs in the

must simultaneously b e dominated by their central re

m range and the underlying continuum at mis

gions of bar typ e SB and of morphological typ e

comparatively very low We conclude that in these galax

earlier than Sb However the reverse is not true as can b e

ies the contribution of nonstellar heating to the emission

seen in Fig NGC a Markarian galaxy

observed inside R is small

CNR

and for instance fulll these conditions b etween

The cases of NGC and NGC can only b e

and of their m radiation comes from small central

discussed on the basis of imaging results The central con

regions resp ectively and of the optical diam

densation of NGC is large wehave determined a

eter yet their F F color is very similar to that of

diameter of kp c and extremely smo oth much

diskdominated galaxies This suggests that they host at

atter than the p oint spread function we therefore con

their center larger concentrations of gas and dust than in

ely a ma jor contribution from the LINERSeyfert sider unlik

the average of galaxies of the same Hubble typ e but for

nucleus which should manifest itself as a p oint source For

some yet undetermined reasons they presently undergo

NGC we cannot conclude and the activenucleus may

smo oth star formation instead of a nuclear starburst We

b e dominant We can only mention that its global color

prop ose that either the net gas inow rate to the center

is lower than that of VCC NGC and this is

has decreased due to a slower replenishment from the

marginally true as well for the nucleus and that the nu

inner disk whichwould have b een previously partially de

cleus of VCC is not classied as active

pleted in gas or a smaller eciency of the evolved bar to

Hence the presence of Seyfert nuclei do es not mo d

make gas lose its angular momentum or since star for

ify our interpretation that high midinfrared colors in the

mation bursts o ccur on a much shorter timescale than bar

present sample are not due to dust heated by nonstellar

life that we are imaging these ob jects at a p erio d of qui

photons and should rather signal the existence of central

escence inb etween bursts Concerning this last p oint see

starbursts

the results of the simulations of Martinet Friedli

and the p opulation synthesis estimates of Kotilainen et al

for the circumnuclear rings of NGC and

Circumnuclearstarbursts

Wenow examine the most likely cause of the m emis

Origin of the circumnuclear infrared excess

sion excesses detected in our sample central starbursts

triggered by the bar dynamical eects Wewarn that

Nonstellar activity

NGC and NGC should b e considered apart

Of the SB galaxies four are known to host a Seyfert

their dust emission comes almost exclusively from cen

nucleus in order of decreasing ux fraction from the

tral regions of kp c but this is more likely due to a

central condensation VCC NGC NGC

past merger than to the inuence of the bar whichmay

NGC and NGC For these the high central color

have b een formed or transformed simultaneously as the

could arise from dust heated by nonstellar radiation from

starburst eventwas triggered

the accretion disk and halo of the central ob ject and would

thus not necessarily indicate the presence of massive stars

Farinfrared diagnostics of nuclear activity are ambiguous

For NGC wehave the direct visual evidence that

for NGC its infrared to radio ux ratio as dened by

the contribution from the activenucleus to the circumnu

Condon et al is q and its sp ectral index b etween

clear emission is negligible since the central midinfrared

and m de Grijp et al is Both values in

source is resolved into the wellknown starforming ring

dicate that stellar and nonstellar excitations may contribute in

whichisvery bright and a faint p oint source at the nu

comparable amounts to the infrared energy output Concerning

cleus Correcting the images of NGC for dilution ef

VCC its sp ectral index b etween and m

fects with a pro cedure analog to CLEAN see the Atlas is rather typical of starbursts

H Roussel et al Impact of bars on MIR dust emission of spirals

molecules m K km s Strong et al Available moleculargas

has b een used to compute the mass as

To see if a signicant dierence exists b etween the central

Z

molecular gas content of circumnuclear starburst galax

M m f T dV K km s D

H H b S

ies and quiescent ones wehave searched the literature for

singledish CO data in the smallest p ossible b eams

exp

CNR S

Singledish data are b etter suited to our purp ose than

interferometric data since the latter are scarcer and do

where m is the hydrogen atom mass and D the distance

H

not collect all the emission from extended structures The

in m In the ab ove formula we estimate the mass only

conversionofCOantenna temp eratures to molecular gas

inside the angular radius used for the infrared pho

CNR

masses is approximate for two main reasons the H mass

tometry of circumnuclear regions Only when the central

to CO luminosity ratio varies with metallicity and physi

regions are resolved and mapp ed is there no need to as

cal conditions and the derivation of CO uxes requires the

sume a brightness distribution H masses derived in this

knowledge of the source structure b ecause it is coupled to

way are probably not more precise than by a factor three

the antenna b eam to pro duce the observed quantity which

including the disp ersion of the factor f but the dynamic

is the antenna temp erature

range in the sample is still sucienttoallow a discussion

Sensible constraints on the structure of CO emission

of the results

can b e drawn from that observed in the midinfrared As

Although the b eam of CO observations is in general

dust is physically asso ciated with gas the midinfrared

larger than it remains except for NGC smaller

CNR

emission spatial distribution should follow closely that

than the diameter of the bar which collects gas from inside

of the gas but b e mo died by the distribution of the

corotation b elieved to b e lo cated close to the end of the

starforming regions that provide the heating and which

bar Athanassoula so that it is still meaningful to

are likely more concentrated than the gas reservoir Since

compare our measurements on infrared condensations to

gaussian proles provide an acceptable description of most

CO data

infrared central regions at our angular resolution wehave

Fig a shows the variation of the m surface bright

therefore assumed that the CO emitting regions are of

ness as a function of the average molecular gas surface

gaussian shap e with halfp ower b eam width HPBW b e

density inside R Higher densities of the molecular

CNR

tween one and two times that at m The m HPBW

material are asso ciated with an increase in the infrared

were derived by matching gaussian proles convolved with

brightness of the central regions This is exp ected since

the p oint spread function to the observed m proles

the amount of dust scales with that of gas which essen

To nd the meaning of various antenna temp eratures

tially consists of the molecular phase in central regions of

with various corrections and whichconventions are used

galaxies More interesting is Fig b where weshow the

in the literature the explanations of Kutner Ulich

evolution of the F F color inside R as a function of

CNR

and Downes were of much help Wecon

the same quantity as in Fig a For the ma jorit y of our

verted given temp eratures to the T scale We then at

R

sample F F tends to rise within a very large disp er

tempted a correction of antenna to source coupling as

sion when the molecular gas mean density increases it

suming a gaussian source and a gaussian diraction pat

roughly doubles when the H surface brightness varies by

tern with angular standard deviations and The

S B

dex However a few galaxies dramatically depart from

relationship b elow follows for the source brightness tem

this trend for colors higher than log F F

p erature T whichisaveraged over the b eam in the ob

b

there is a reversal in the sense that hot circumnuclear re

servation whereas wewanttorecover its intrinsic value

gions seem to b e depleted in molecular gas with resp ect

over the source extent

to the normal H contentcolor distribution

T T

b

S B S R

Although one can think of several reasons why their

Table contains the b eam width of the observa molecular contentmay b e underestimated the standard

conversion factor may not apply for these galaxies due to

tions and the derived H masses for the adopted ref

ICO erences A conversion factor f N H their starburst nature or p ossibly due to a lower metal

licity it is unlikely that this is the case First the im

In NGC the scales of the molecular gas and infrared

plied underestimation factors app ear quite large at least

concentrations are of the same order In NGC the source

to Second if wewere to correct the H masses by

HPBW is estimated to b e whichis times that

these factors to bring the galaxies within the trend ob

of the innermost infrared regions However these are clearly

served in Fig b then these ob jects would b ecome ab

more extended than a central gaussian and not representative

normal in Fig a with a decit of m emission The

of normal CNRs since likely gathered by a merger For this

and other galaxies whose central regions are clearly structured four deviating galaxies do not share a common prop erty

ie NGC and NGC detailed CO maps where the

whichwould make them sp ecial with resp ect to all the

source is resolved were used It is also the case for NGC

others NGC and IC are similar SBd galaxies

and



Note that it is however conceivable that a fraction of UIB which includes corrections for atmospheric attenuation and

carriers are destroyed whichwould cause such a decit all instrumental eects except antenna to source coupling

H Roussel et al Impact of bars on MIR dust emission of spirals

densityb ounded is made see Whitworth also for

a discussion of the eciency of molecular cloud disp ersal

byyoung stars Dust should then b e depleted to o how

ever b ecause of the presence of massive stars the remain

ing dust is exp osed to a very intense radiation eld and

F color This ratio may also increase reaches a high F

due to the fact that the dust whichwas mixed with rather

dense molecular clouds of low F F color has b een dis

p ersed to o Alternatively the concentrations of molecular

gas in these galaxies may b e more compact than in the

others and diluted in our large b eam we cannot exclude

that the midinfrared distribution includes an unresolved

core which dominates the color A conrmation of the

ab ove scenario clearly requires b etter measurements of the

central gas content and highresolution characterization of

the starbursts

Leaving the four galaxies in the upp er left quadrantof

Fig b apart the data supp ort an interpretation in terms

of starburst with standard prop erties the infrared activ

ity in galactic centers can b e stronger when the available

molecular gas is denser

Color of the central concentration and age of

the starburst

Fig indicates howthe F F color inside R varies

CNR

with the m surface brightness in the same ap erture

In principle the midinfrared surface brightness can in

crease either b ecause the amount of dust in the considered

area is higher suchasobserved in Fig a or b ecause

the energy densityavailable to heat the dust increases

The trend for higher F F ratios at large msurface

brightnesses seen in Fig indicates that indeed the in

crease of the m surface brightness is at least partly

Fig a m surface brightness as a function of the aver

due to rise of the mean energy density in the CNR In this

age H surface density b oth inside the circumnuclear regions

diagram again the galaxies with a p eculiar b ehavior in

dened by midinfrared photometry CNR The limits on H

ythe Fig b stand apart well ab ove the lo cus dened b

mass are not true error bars but simply indicate the eect

least absolute deviation t dashed line This supp orts

of varying the scale of the gaussian distribution from once to

the fact that the trend seen in Fig b is not due to an un

b F F twice that measured at m see text color as

 

a function of the average H surface density b oth inside the

derestimation of the H content and lends further credit

a CNR with the same convention for error bars as in

to the interpretation presented in Sect

Another study by Dale et al has already dealt

with the jointvariations of midinfrared surface bright

VCC NGC is a small and lowluminosity SBa

nesses and colors However contrary to Fig where the

and VCC NGC is an edgeon Seyfert SBab for

surface brightnesses and colors are those of the same phys

which the molecular contentmay b e illdetermined due to

ical region the CNR in a large sample of galaxies in the

the integration of the CO line throughout the disk

Dale et al study resolution elements inside the

Wethus prop ose the following interpretation for the

target galaxies are rst binned according to their surface

galaxies that wander o the main trend in Fig b the

brightness b efore the mean color of the bin is computed

main distribution corresp onds to galaxies where the cen

As a result a bin do es not corresp ond to a physical ob ject

tral starburst is more and more intense as indicated by

We simply note that if galactic central regions are binned

the high gas surface densities and colors Galaxies at the

by surface brightness in Fig then the obtained mean

turnover of the sequence may b e observed in a phase of

lo cus is comparable to those shown by Dale et al

their starburst not necessarily common to all galaxies



when it has consumed or disp ersed most of the accumu

Wegive this name to a t where the quantitytobemin

lated gas b ecause of a higher star formation eciency

imized is the sum of absolute values of the distances b etween

This suggests an interesting analogy with HII regions for

the data p oints and the line This metho d is less sensitiveto

outliers than the least squares t which the distinction b etween ionizationb ounded and

H Roussel et al Impact of bars on MIR dust emission of spirals

Using the p opulation synthesis results of Bonatto et The galaxies with the highest central F F colors

al based on ultraviolet sp ectra b etween and F F and which stray from the main trend

A we can estimate the mean stellar age in the central are barred but their bars are of mo derate lengths once

weighted by the fraction of luminosity emitted depro jected and normalized by the optical diameter

at Aby dierent p opulation bins This was p ossi In NGC NGC VCC NGC and

ble for eleven galaxies of our sample in common with the IC for which it could b e estimated D D

bar

sample of Bonatto et al We compare in Fig when this ratio ranges b etween and in galax

this mean age to the F F color deviation dened as ies with measurable bar length This conrms that the

the dierence b etween the observed color and that pre central activity signalled by a high F F color is not

dicted by the mean distribution of all galaxies indicated an increasing function of bar strength as can b e exp ected

by the least absolute deviation t in Fig at the same from the dierent timescales for star formation and bar

m surface brightness For this purp ose we p erformed evolution

the midinfrared photometry in a slit ap erture identical to

Since the bar strength alone is not sucient to ex

that used by Bonatto et al We indeed see that the

plain the observed midinfrared colors and since the ob

y ounger the weighted age the higher the central F F

servational uncertainties are much smaller than the scat

color deviation This is thus in agreement with our hy

ter present in Fig one may susp ect that part of this

p othesis that much of the color variations in Fig may

scatter is due to intrinsic prop erties of each of the cir

beduetoagevariations of the exciting p opulations

cumnuclear starbursts considered Indeed given that mid

infrared emission likely traces star formation on timescales

There are grounds to think that some scatter in Fig

longer than for instance recombination lines it is reason

is due to the metho dology adopted by Bonatto et al

able to exp ect that for similar midinfrared brightnesses

in their study They have group ed galaxies of their

corresp onding to similar gas and energy densities the

sample according to sp ectral resemblance morphological

midinfrared color could vary as a function of the age

typ e and luminosity and coadded all UV sp ectra of each

of the stellar p opulations resp onsible for dust excitation

group in order to increase the signal to noise ratio b efore

Since star formation do es not happ en instantaneously all

p erforming the p opulation synthesis Howeveritmaynot

through a kp c region and likely o ccurs in cycles trig

b e fully justied to average sp ectra of dierent galaxies

gered by instabilities these stellar p opulations are multi

with the same overall shap e but dierent sp ectral signa

ple and their ages should b e weighted to reect the suc

tures A further drawback of this study is that it cannot

cessive generations of stars contributing to dust heating

prop erly takeinto account extinction b ecause of the lim

itation to a small sp ectral range in the UV the derived

very low extinctions are meaningless That is why the two

galaxies departing from the welldened trend describ ed

ab ove could owe their age to the metho d rather than to

their intrinsic prop erties

VCC NGC is assigned the same large

weighted age as NGC their UV sp ectra are co

added but has a higher color excess with resp ect to

the mean distribution in Fig In fact Maoz et al

detected very strong P Cygni absorption lines

of highexcitation ions CIVSiIVNVcharacteristic

of the winds of massiveyoung stars Myr and its

sp ectrum b etween and A is nearly identical

to that of the starburst NGC B

h results in NGC is coadded with NGC whic

a small weighted age Yet its color excess is muchlower

than that of NGC and more comparable to that of

NGC There is some evidence that the extinction

Fig Variation of the midinfrared color with the m sur

in the central regions of NGC is muchlower than

face brightness in mJy arcsec b oth inside the same ap er

in other galaxies whereas available Balmer decrement

ture centered on circumnuclear regions The mean error bar is

measures indicate H absorptions of the order of

shown in the upp er left corner The average lo cation of disks

in other nuclei the decrement given by Diaz et al

in the diagram in terms of average color and global surface

for NGC indicates AH
Even if

brightness the disk area b eing delimited by the blue isophote

the Balmer decrement is not a go o d extinction mea

mag arcsec would b e at

The dashed

B

line represents the formal least absolute deviation t p er

sure it is instructive to compare values in dierent

formed includin g all the galactic central regions used to dene

galaxies We also notice that NGC is the only one

the color deviation in Fig

among strongly barred spirals which has an amorphous

H Roussel et al Impact of bars on MIR dust emission of spirals

gering of intense star formation so that the interstellar

radiation eld increases reected in higher F F ratios

Fig and suggest then that deviations from this sim

ple description can b e related to the star formation history

of the circumnuclear regions Ongoing starbursts pro duce

excess F F colors while faded starbursts are asso ciated

with F F decits

Additional variation in midinfrared colors may arise

from dierences in metallicity and in the compactness of

the starburst with consequences on the amount and na

ture of the dust but this is out of the scop e of the present

study

Summary and conclusions

Wehave studied the midinfrared activity induced by bars

in a sample of nearby spiral galaxies with infrared lumi

Fig The abscissa indicates the mean age of stellar p op

nosities spanning a large range b elow the class of luminous

ulations according to the synthesis results of Bonatto et al

infrared galaxies Wehave found that

including the rst six elements of their base stellar

clusters to which they attribute ages b etween and Gyr for

The midinfrared emission of the normal galaxies in

the rst ve and in the interval Gyr for the last one but

our sample is essentially contributed by a thermal con

excluding the oldest element an elliptical bulge representing

tin uum from very small grains VSGs longward of

ages b etween and Gyr The ages are weighted by the frac

m and the family of aromatic bands UIBs de

tion of the ux at A that each dierent p opulation emits

tected in a wide diversityofenvironments It is the

The contribution from continuous star formation has b een ap

variation of the VSG comp onent with resp ect to the

proximated by a constant ux fraction equal to the minimum

UIBs that is resp onsible for F F changes in our

value and subtracted in order to consider only successions of

galaxies From the comparison with observations of

bursts The ordinate is the dierence b etween the measured

resolved Galactic regions this can b e related to an

F F color and the color exp ected from the mean relation



increase of the lling factor of star forming complexes

ship b etween central surface brightnesses and colors shown in

Fig For this graph the photometry was p erformed inside

by photoionized regions hence a decrease of the contri

the same ap ertures as in Bonatto et al and

bution to the midinfrared emission from neutral and

error bars show the eect of varying the slit orientation which

molecular media

is not given by Bonatto et al

There is a dichotomybetween spiral disks where the

integrated F F color is close to and shows little

disp ersion and circumnuclear regions where F F

circumnuclear region with no hot sp ot that would in

ranges from disklike to high valuesuptoWehave

dicate the presence of massive stellar clusters

found no indication that destruction of UIB carriers

o ccurs at the scale of circumnuclear regions although To conclude Fig is a strong indication that the mid

it would b e desirable to analyze infrared sp ectroscopic infrared emission in circumnuclear regions is inuenced by

data of the most active galaxies to elab orate on this successive episo des of star formation over relatively long

We conrm that barred spirals distinguish themselves p erio ds of time on the mean the F F color is a sen

from unbarred galaxies in the sense that they can sitive function of the midinfrared surface brightness but

reach higher F F colors This eect is however re this relationship is mo dulated by the mean age of the stel

stricted to early morphological typ es in agreement lar p opulations A strinking example of this is NGC

with previous IRASbased studies Hawarden et al Its central midinfrared brightness is in the high range

tral F F ratio is low in accordance with Huang et al Weshow unambiguously but its cen

its large mean stellar age conrmed byTaniguchi et al that this emission excess arises in circumnuclear re

From optical p opulation synthesis they nd that a gions which can completely dominate the midinfrared

central starburst o ccurred ab out Gyr ago in this galaxy emission although their size remains mo dest D

CNR

and that subsequentnuclear star formation has pro ceeded ranges b etween and of the optical diameter

atalow rate in the whole sample with no clear dep endency on

Combining this result with that of Sect wecan Hubble typ e Galaxies with a global color excess are

tral regions This is a con form the following sketch of what determines the mid all dominated by their cen

rmation of predictions from hydro dynamical mo dels infrared prop erties of circumnuclear regions the central

Athanassoula Friedli Benz according surface brightness is connected to the amountofgas

to which a barred p erturbation through tidal torques as exp ected if gastodust ratios are relatively constant

and sho cks induces substantial mass transfer towards However accumulation of gas in the center allows the trig

H Roussel et al Impact of bars on MIR dust emission of spirals

Acknow ledgements We thank our referee Louis Martinet for circumnuclear regions We observe the consequences

his helpful remarks

of these gas ows ie the intense star formation that

they fuel

The ISOCAM data presented in this pap er were analyzed using

An imp ortant fact to mention is that only a fraction

and adapting the CIA package a jointdevelopmentbythe

ESA Astrophysics Division and the ISOCAM Consortium led

of earlytyp e barred galaxies can b e distinguished from

by the PI C Cesarsky Direction des Sciences de la Matiere

unbarred galaxies in their infrared prop erties Several

CEA France

interrelated parameters may explain this quiescence of

many barred galaxies With the present data weare

unable to address this issue and thus only list p ossible

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Table General prop erties of sample galaxies Virgo memb ers are also named from the VCC catalog Other galaxies b elong

to the eld or lo ose groups unless otherwise noted Distances are from the NGC catalog of Tully taking into account

the Virgo infall and assuming h morphological typ es asymptotic blue magnitudes m and ma jor diameters D at

BT

the isophote mag arcsec arefromtheRC de Vaucouleurs et al

B

a b c

name RA DEC D morph m D HI def M b nuclear tidal

BT H

Mp c typ e log M typ e int

N SBb c P LHII V

N SBd T E HII V mbs

N SBb c MF E LHII V

N SBa TB GD HII A am

N SBb MF GN LSy VM doa

N y SBb J SJ Sy VM

N z SBab RB BW LSy VM

N SBb RT RD

N z SBb MP BW LHII V

N SBdm RH HII V mbs

N V SABc GR HII V

N SBa HS W HII K am

N M SAab HS ST L H

N M SAb c AF HB Sy H doa

N M SABc HB HN HII V

N Mrk SBb S SR HII A

N M SABcd HB SI HII H doa

N SABb c MF L V

N x SBab T CS HII V

Virgo cluster sample

N V SBdm GR HII H

N V SABab GR KY LHII H

N V SBa GR KY L K

N V SBab GR note

N V SBab GR V Sy K

N V SBb GR KY L K

N V SBab GR

N V SBb HH doa

N V SAa GR CD L K pc

N V SAab GR Bo L K

N V SBa GK Bo

N V SABd GR

N V SBa HG

N V SAb c j C KY HII H

N V SAb c j E HII K

N V SABab GR Bo LHII H

N V SABb GR E LSy H

N V SABa HH

N V SABdm HH

N V SBcd HH

N V SABc GR KY HII K

N V SABcd GR Br HII V doa

N V SAb c GR HII K

H Roussel et al Impact of bars on MIR dust emission of spirals

Table continued The last column indicates the name of the observation pro ject S for Sf glx or HI Q gal for IC PI G

Helou and I for Irgal PI T Onaka

a b c

name RA DEC D morph m D HI def M b nuclear tidal

BT H

Mp c typ e log M typ e int

Supplementary galaxies

N SBab RM E HII V S

N y SB HC W L MC S

N SBcd AH AC HII V mbs S

N SBb T SS HII H S

d

N SBab RM E S

N SBc Ka S

N SABab HZ LHII H S

N SAa C S

N SAb c T E HII H I

N SABb T M HII H S

N V SBd GR Y S

N SABd HZ LHII H S

d

N Mrk SBb TB KS HII K I

d

N SBb c RM S

d

N SABb TB LHII A I

N SAc Kr HII V S

d

N SABc I

d

N SAb S

d

N SAb K I

N SABcd RS HII H S

N SBcd T S

N SABcd T S

d

N Mrk SBa MS Y HII VK S

IC SBd T CP S

IC SBb c RC AB doa I

IC SABb c T S

d

ESOG SBa S

a

The HI deciency according to the denition and reference values of Guiderdoni Ro cca It is normalized by the

disp ersion in the eld sample Diameters are taken from the RC for consistency and HI uxes from the indicated references

NGC are unresolved in HIWhen D D and Def inCayatte et al it corresp onds to def

HI opt

here

b

The given range represents the eect of varying the scale length of the CO distributio n from once to twice that of infrared

circumnuclear regions see text b is the b eam HPBW of the observations used NGC was observed by A Bosma D

Reynaud and H Roussel at the IRAM m telescop e

c

Signs of tidal interaction doa asymmetrical distortion of outer arms mbs magellanic barred spiral am amorphous

p c past collision note On DSS images the brightness p eak is displaced by ENE from the center of the regular

outer isophotes and the NW outer disk seems depressed in stars and gas

y memb ers of the Fornax cluster of galaxies z memb ers of the Dorado group of galaxies x memb er of the Grus quartet with

NGC memb er of the spiralrich Ursa Ma jor cluster

classied SA in the RC here considered a SB after the morphological arguments of Phillips Malin and McLeo d

Rieke and the recent kinematic analysis of Veilleux et al

d

The distances of ESO G NGC and were assumed to b e those of the galaxy groups LGG LGG

and LGG Garcia those of NGC and were estimated from the HI redshift that of NGC from the CO

redshift and those of NGC and from the optical redshift with h

H Roussel et al Impact of bars on MIR dust emission of spirals

Table Photometric results at and m obtained as describ ed in the Atlas total uxes diameter ap erture used for central

regions uxes inside this ap erture and background levels Wewarn the reader that the uncertainties can only b e taken as order

ofmagnitude values see the text esp ecially for galaxies of the third subsample b elonging to the Sf glx pro ject with a very

lownumb er of exp osures p er sky p osition Galaxies with no rep orted central uxes have no identiabl e central concentration

the radial surface brightness prole is consistent with a disk alone at our angular resolution NGC rather shows a smo oth

central plateau and NGC is seen edgeon For NGC we used only the maps with a pixel size b ecause those at

are strongly saturated in b oth lters for the other galaxies mapp ed with b oth pixel sizes we used the sampling b ecause of

the higher signal to noise ratio and the more reasonable eld of view

name F F D F F b b

tot tot CNR CNR CNR

a a

mJy kp c mJy Jy arcsec

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

Virgo cluster sample

N

N

N 

N

N

N

N

N

N 

N 

N

N

N

N y

N y

N

N

N

N

N

N

N

N

H Roussel et al Impact of bars on MIR dust emission of spirals

Table continued

name F F D F F b b

tot tot CNR CNR CNR

a a

mJy kp c mJy Jy arcsec

Supplementary galaxies

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

I

I

I

ESO

a

The conversion from ux densities to uxes is F W m F Jy THz with the lter widths

THz and THz

nucleus saturated for NGC b oth at and m but more severely at m since the same gain and integration

time were used for b oth lters and since F F is ab ove in electronic units for NGC slightly at m but not at m

for the central pixel of NGC at m but not at m for NGC at m but not at m which is p ossible b ecause

the integration time was resp ectively s and s for NGC and b oth at and m but more severely at m with

the same conguration as NGC Thus F F colors in central regions are resp ectively lower limits for NGC

and and upp er limits for NGC and

The eld of view is to o small to allow a precise determination of the backgroud level and total uxes are lower limits

The error bars are only formal The comparison of our measurements with those of Rice et al at m inside the IRAS

hoverlaps with our m and m bands indicates that we miss of the order of of total uxes band m whic

for NGC and b etween and for NGC provided IRAS uxes are not overestimated as this is often the case for

coadded observations


From their sp ectral energy distribution s shown by Boselli et al these galaxies probably have a nonnegligib le

contribution from the RayleighJeans tail of cold stars to their m emission We did not attempt to remove this contribution

b ecause it would require a careful mo delling of stellar p opulation s

y The disks of these galaxies slightly overlap in pro jection We attempted to separate them by the means of a mask dened

visually but the disk uxes are much more uncertain than estimated VI. Activit´e de formation d’´etoiles au centre des galaxies barr´ees 169

VI.2 Taille et brillance des r´egions centrales

Dans l’article qui pr´ec`ede, nous nous sommes content´es de souligner que l’activit´edeforma- tion d’´etoiles d´epend de l’existence d’une barre, mais n’est pas corr´el´ee avec la “force” de la barre, ainsi que j’appelle par abus de language la taille d´eprojet´ee de la barre normalis´ee par −2 la taille du disquea ` l’isophote µB = 25 mag.arcsec . Examinons un peu plus en d´etail les param`etres de taille des barres et des r´egions centrales.

La taille de la barre a pu ˆetre estim´ee sur les images optiques de 14 galaxies des ´echantillons de Cambarre et Camspir, qui ne sont pas trop inclin´ees et dont la barre est suffisamment bien d´efinie en brillance, et a ´et´ed´eprojet´ee par la relation  D D / − θ i 2 bar =( bar)obs 1 (sin sin ) (VI.1) o`u θ est l’angle entre la ligne des nœuds et l’axe majeur de la barre et i l’inclinaison du plan de la galaxie sur la ligne de vis´ee. L’angle de position de la ligne des nœuds et l’inclinaison proviennent d’analyses cin´ematiques si elles existent, et sinon des param`etres isophotaux donn´es dans le catalogue RC3. La figure VI.1a montre que dans le sous-´echantillon de spirales consid´er´e, les r´egions centrales o`u se concentre la formation d’´etoiles tendent `as’´etaler lorsque la barre est plus grande. Cela se con¸coit assez facilement si l’on suppose qu’une barre plus grande est, du fait de l’extension de la zone qu’elle balaye et de sa force dynamique, capable d’amener plus de gaz vers les r´egions centrales. Dans les galaxies fortement barr´ees du sous-´echantillon de la figure VI.1a, la forme des bandes de poussi`ere indique que des r´esonances internes de Lindblad existent (Athanassoula 1992b) ; cet argument est renforc´e par le fait que toutes ces galaxies poss`edent des spirales ou anneaux circumnucl´eaires (voir l’Atlas). L’existence et l’extension des r´esonances internes de Lindblad d´ependenta ` la fois de la concentration de masse centrale et la vitesse de rotation de la barre. On comprend ainsi pourquoi les r´esonances internes de Lindblad (qui d´eterminent la taille des r´egions circumnucl´eaires o`u s’accumule le gaz) doivent croˆıtre lorsque la barre est plus grande : du fait qu’une barre se termine peu avant la corotation (Athanassoula 1992b), les barres longues doivent avoir tendancea ` tourner moins vite, et comme elles drainent plus de gaz vers le centre, elles augmentent la masse centrale. La taille des barres, par ailleurs, tendaˆ ` etre corr´el´ee avec la taille des bulbes (Athanassoula & Martinet 1980), ce qui va dans le mˆeme sens d’une augmentation de la masse centrale.

Dans la figure VI.1b, j’ai ajout´e les 12 galaxies des programmes Sf glx et Irgal pour lesquelles la taille de la barre peut ´egalement ˆetre estim´ee. Ces galaxies introduisent essentiellement une grande dispersion. Mˆeme si les mesures de taille sont tr`es incertaines, cette dispersion est certainement r´eelle, en partie pour la mˆeme raison qui nous a fait remarquer que l’activit´ede formation d’´etoiles n’est pas corr´el´ee `a la force de la barre. En effet, la croissance des r´egions centrales est un ph´enom`ene irr´eversible, qui implique qu’elles restent brillantes en infrarouge moyen apr`es qu’un sursaut de formation d’´etoiles se soit d´evelopp´e. La force de la barre,a ` l’inverse, ´evolue au cours du temps et peut en particulier se r´eduire suitea ` un sursaut central de formation d’´etoiles (Martinet & Friedli 1997). Une telle ´evolution aura pour effet de faire se d´eplacer les galaxies vers la gauche du diagramme VI.1.

La figure VI.2 montre comment se distribuent la taille des r´egions centrales observ´ees en 170

Figure VI.1: Variations de la taille des r´egions centrales, d´efinies par l’ajustement des profils de brillancea7 ` µm, avec la taille de la barre, toutes deux normalis´ees par le diam`etre du disque −2 `a l’isophote µB =25mag.arcsec . Les points noirs marquent les galaxies classifi´ees SB dans le RC3, et les cercles avec croix les galaxies classifi´ees SAB. infrarouge moyen et la taille de la barre en fonction du type morphologique, pour les mˆemes galaxies que ci-dessus. On constate comme on pouvait s’y attendre que les param`etres de taille ne sont que faiblement corr´el´es au type morphologique. Cependant, les galaxies barr´ees pr´ecoces se situent exclusivement dans le domaine des grandes barres et des r´egions centrales larges. En revanche, les galaxies faiblement barr´ees et non barr´ees ne montrent aucune relation bien affirm´ee entre la taille des r´egions centrales infrarouges et le type.

Venons-en maintenanta ` l’effet des param`etres de taille sur l’activit´e de formation d’´etoiles. Dans notre ´echantillon, la brillance de surfacea7et15 ` µmdesr´egions circumnucl´eaires est ind´ependante de leur taille. Par ailleurs, on se rend clairement compte dans Fig. VI.3 que les VI. Activit´e de formation d’´etoiles au centre des galaxies barr´ees 171

Figure VI.2: Taille normalis´ee des r´egions centrales infrarouges (en haut) et de la barre (en bas) en fonction du type morphologique. couleurs centrales les plus ´elev´ees, manifestant l’activit´e de formation d’´etoiles la plus intense d’apr`es l’interpr´etation propos´ee dans l’article pr´ec´edent, se trouvent dans des galaxies dont la barre est de force mod´er´ee et les r´egions centrales relativement petites. Cela confirme la d´ecorr´elation entre les param`etres dynamiques et les indices de formation stellaire, qui ´evoluent sur des ´echelles de temps tr`es diff´erentes.

La figure VI.5, o`usonttrac´ees les brillances de surfacea7et15 ` µmdesr´egions centrales, montre que la brillance attribuable aux bandes aromatiques (`a7µm) n’augmente pas aussi vite que la brillancea15 ` µm, o`usontm´elang´es les bandes aromatiques et le continuum thermique ≈ 0.86 des tr`es petits grains (Σ7 Σ15 ). Cet effet, o`u une variable cach´ee (difficile `a estimer) est la densit´e de rayonnement, explique une partie des couleurs ´elev´ees observ´ees dans les r´egions centrales. Cependant, de nombreuses galaxies sortent de la corr´elation dans ce graphique. Cela 172

Figure VI.3: Couleur F15/F7 des r´egions centrales en fonction de la taille de ces mˆemes r´egions (en haut, pour toutes les galaxies qui poss`edent une r´egion circumnucl´eaire s´eparable du disque) et de la taille de la barre (en bas, pour les galaxies dont la barre est mesurable). ad´ej`a´et´ediscut´e dans l’article qui figure au d´ebut de ce chapitre. Dans le cas des quelques galaxies “anormales” qui poss`edent les couleurs F15/F7 les plus ´elev´ees, on peut avancer deux hypoth`eses : soit le facteur de remplissage du milieu interstellaire par les r´egions HII est plus grand qu’au centre des autres galaxies, ce qui a pour effet d’augmenter l’´emission des tr`es petits grains relativement aux porteurs des bandes aromatiques, soit ces porteurs de bandes ont ´et´e pr´ef´erentiellement d´etruits par un rayonnement ultraviolet dur ou dans des chocs de supernovæ. Il faut aussi noter que si l’´emission provient essentiellement de condensations isol´ees (des “hot spots”), elle est dilu´ee `al’´echelle des r´egions que l’on peut d´elimiter sur les cartes, et les brillances de surface sont alors fortement sous-estim´ees.

En conclusion, on observe une d´ecorr´elation entre les param`etres de taille de la barre et des VI. Activit´e de formation d’´etoiles au centre des galaxies barr´ees 173 r´egions centrales infrarouges d’une part, et entre les indicateurs de formation stellaire et ces param`etres de taille d’autre part (Fig. VI.3 et Fig. VI.4), ce qui est explicable par de simples arguments de temps caract´eristiques et d’´evolution. La taille de la barre ´evolue sur des ´echelles de temps de l’ordre de 109 ans, la taille des r´egions centrales, qui ne peut que croˆıtre, d´epend de toute l’histoire pass´ee de la galaxie et l’activit´e de formation d’´etoiles est br`eve et ´episodique.

Figure VI.4: Brillance de surfacea15 ` µm(tra¸cant la densit´e de formation d’´etoiles) des r´egions circumnucl´eaires et taille normalis´ee de la barre.

Figure VI.5: Brillances de surfacea15et7 ` µm dans les r´egions circumnucl´eaires (NGC 6744 exclue). L’ajustement robuste (minimisation des distances en valeur absolue) donne une loi de puissance en 0.86 . 174 175

Chapitre VII Conclusions et poursuite envisageable

La cartographie en infrarouge moyen d’un large ´echantillon de galaxies spirales, s´electionn´ees pour ˆetre de luminosit´e infrarouge mod´er´ee, ne pas subir d’interaction gravitationnelle forte et ne pas d´eployer d’activit´e de Seyfert intense, a produit les r´esultats suivants :

— La morphologie des galaxies vues au travers de l’´emission de deux phases de poussi`ere, les porteurs des bandes aromatiques et les tr`es petits grains carbon´es, est remarquablement semblable `a la morphologie du rayonnement stellaire dans le bleu ou le rouge, et encore plus de l’´emission n´ebulaire dans la raie Hα.L’´emission `a7et15µm fait ressortir des bras spiraux fins et des r´egions circumnucl´eaires de 1 `a3kpcdediam`etre, en g´en´eral tr`es brillantes. On note aussi une ´emission diffuse (assez difficile `a quantifier du fait de la r´esolution angulaire mod´er´ee), une absence de bulbe, et une ´emission en g´en´eral faible le long des barres, d´ecal´ee du cˆot´e aval, avec quelques complexes brillants dans la partie externe de la barre. Les galaxies particuli`eres amorphes constituent une exception notable `a cette morphologie d’ensemble, leur ´emission infrarouge ´etant extrˆemement concentr´ee.

On note un effet d’environnement dans les galaxies membres de l’amas de Virgo, une r´eduction de la taille des disques ´emettant en infrarouge moyen par rapport au disque optique lorsqu’une d´eficience en gaz HI est observ´ee. Dans un cadre plus g´en´eral, cet effet, visible aussi dans les galaxies dont les populations stellaires peuplant le disque externe sont d’aspect t´enu ou ´evolu´e, est compr´ehensible par l’existence d’un seuil en densit´ede colonne de gaz pour la formation d’´etoiles. Un autre effet d’environnement, incertain, reste `a confirmer : l’activit´e centrale de certaines galaxies ayant interagi avec le gaz intra-amas semble ˆetre particuli`erement intense, plus que dans les galaxies barr´ees normales.

—L’´emission dans les bandes aromatiques, int´egr´ee sur toute une galaxie, fournit une es- timation valable du taux de formation d’´etoiles dans le cas des galaxies spirales peu ac- tives tout comme dans des galaxies starbursts telle M 82. Cela va `a l’encontre de ce qui est g´en´eralement admis, les flux dans la bande d’IRASa12 ` µm, qui contient des massifs de bandes aromatiques, ´etant classiquement associ´es, pour une large fraction,adessites ` d’´emission excit´es par un rayonnement peu ´energ´etique.

Al’´echelle des r´egions HII Galactiques, des observations ant´erieures ont mis en ´evidence une destruction des porteurs de bandes aromatiques dans des champs de rayonnement de tr`es grandes densit´es d’´energie. De ce fait, on s’attend `a observer une saturation de 176

l’´emission `a7µm dans des galaxies ou des noyaux de galaxies `a sursaut de formation d’´etoiles, o`u le facteur de remplissage du milieu interstellaire par les r´egions HII devient important. Cependant, dans la large gamme dynamique en densit´es de formation d’´etoiles qui a ´et´eexplor´ee, une telle saturation n’est pas observ´ee (du moins pour des galaxies qui ne sont pas sous-m´etalliques).

—Danslesr´egions circumnucl´eaires, une autre composante de poussi`ere est responsable d’une fraction variable du flux collect´e dans l’intervalle 12-18 µm:lestr`es petits grains ´emettant un continuum thermique au-del`ade10µm. Ce continuum apparaˆıt toujours n´egligeable dans l’´emission totale des disques, o`u la bande passante 12-18 µm est alors domin´ee par des bandes aromatiques, et ne se manifeste dans ce domaine de longueurs d’onde,anotre ` r´esolution angulaire, que dans quelques complexes brillants de formation d’´etoiles et dans les r´egions centrales des galaxies. L’´emission des tr`es petits grains, telle qu’´echantillonn´ee par les flux dans une bande ´etroite centr´ee `a15µm, ob´eit `a une relation lin´eaireavecleflux de photons ionisants, pour des densit´es de formation d’´etoiles sup´erieures `a celle observ´ee dans le plateau central de M 51, au niveau de laquelle on observe un d´ecrochement.

Un r´esultat pr´eliminaire indique par ailleurs que la couleur F15/F7 des r´egions circum- nucl´eaires des galaxies spirales d´ependalafoisdeladensit´ ` ed’´energie du rayonnement et de l’histoire de la formation d’´etoiles,a ` laquelle F15/F7 est sensible sur des p´eriodes assez longues (≈ 109 ans).

— Du point de vue de l’activit´e infrarouge int´egr´ee, nous confirmons l’existence d’une di- chotomie entre les galaxies fortement barr´ees de types morphologiques pr´ecoces, qui seules peuvent avoir un exc`es de couleur F15/F7, et les autres galaxies, et nous localisons cet exc`es de couleur dans des r´egions centrales de petite taille (typiquement 1a ` 3 kpc). Les couleurs ´elev´ees des centres galactiques sont dues `a des sursauts de formation d’´etoiles d’intensit´e mod´er´ee, qui sont la cons´equence indirecte des flots de gaz induits par le potentiel d’une barre.

Dans l’´echantillon ´etudi´e aussi bien que dans les plus larges catalogues de galaxies observ´ees par IRAS, une fraction seulement des galaxies SB pr´ecoces montrent une ac- tivit´eparticuli`ere, ce qui se comprend facilement par la bri`evet´edes´episodes de formation d’´etoiles, qui consomment le gaz d’autant plus efficacement qu’il est plus dense, et les ´echelles de temps beaucoup plus longues pour l’´evolution des barres et l’accr´etion de gaz au centre.

—D’apr`es les observations en CO(1-0) que j’ai pu compiler, la densit´e de surface du gaz mol´eculaire disponible dans les r´egions centrales (telles que d´efinies `apartirdeladistribu- tion de brillance en infrarouge moyen) tenda ` augmenter en mˆeme temps que les brillances infrarouges, ce qui peut r´esulter du simple fait que la poussi`ere et le gaz (essentiellement sous forme mol´eculaire dans les centres galactiques) sont intimement li´es.

La quantit´e de gaz mol´eculaire dans la zone circumnucl´eaire est cependant grossi`erement corr´el´ee `alacouleurF15/F7,venant`a l’appui d’une interpr´etation de cette couleur en termes de starburst plus ou moins intense dont le d´eclenchement suit au premier ordre un crit`ere d’instabilit´e du type de celui de Toomre. VII. Conclusions et poursuite envisageable 177

Les quelques centres galactiques ayant les couleurs infrarouges les plus grandes (c’est `adireF15/F7 > 3) semblent d´eprim´es en gaz mol´eculaire. Cela peut ˆetre dˆu`a une sous- estimation du facteur de conversion CO-H2 si ces galaxies sont sous-m´etalliques,a ` une consommation et une dispersion tr`es efficace du gaz par le starburst, ou encore `a une r´epartition du gaz mol´eculaire dans des zones tr`es compactes, dont le rayonnement serait dilu´e`a l’int´erieur de notre ouverture. Les donn´ees dont je dispose ne permettent pas de favoriser l’une ou l’autre de ces explications.

Pour compl´eter ce travail, il serait souhaitable :

— d’obtenir, dans des galaxies dont l’orientation et l’inclinaison sur la ligne de vis´ee sont favorables, des observations du gaz mol´eculaire dans les r´egions centrales, fournissant une mesure `alafoisdesadensit´e de colonne et de la courbe de rotation. Cela permettrait d’une part de comparer la masse gravitantea ` la masse de gaz, de mani`ere `a avoir une id´ee de l’efficacit´e d’accr´etion centrale du gaz par la barre au cours de toute son existence, et d’autre part d’estimer l’efficacit´e de formation d’´etoiles et le param`etre de stabilit´ede Toomre pour le confronteral’´ ` etat de la formation d’´etoiles tel qu’observ´e en infrarouge moyen. Ce serait aussi l’occasion de r´eexaminer, avec une haute r´esolution grˆace aux techniques interf´erom´etriques, le cas des galaxies qui semblent avoir un contenu en gaz mol´eculaire anormal au regard de leur couleur infrarouge.

— de chercher, par une analyse cin´ematique, l’existence et la localisation des r´esonances entre l’onde barr´ee et le potentiel non perturb´e, de fa¸conaexaminerpr´ ` ecis´ement leur rˆole dans la distribution de la formation d’´etoiles.

— d’obtenir des observations spectroscopiques en infrarouge moyen,a ` plus grande sensibilit´e et meilleure r´esolution angulaire, des r´egions circumnucl´eaires et de complexes de forma- tion d’´etoiles dans les barres et les disques d’un ensemble de galaxies bien choisies parmi celles de notre ´echantillon, ce qui sera possible avec IRS, le spectrom`etre `a bord du futur satellite SIRTF, qui op`erera entre 5 et 40 µm et permettra de mieux caract´eriser le contin- uum thermique des petits grains. Il faudrait s´electionner des galaxies d’activit´e infrarouge d’intensit´es tr`es diverses, dont les galaxies `a grands rapports centraux F15/F7 qui semblent avoir un contenu en gaz mol´eculaire anormal. Cela permettrait de v´erifier si le spectre des bandes aromatiques subit des changements ou non dans ces galaxies particuli`eres, de quantifier pr´ecis´ement le continuum thermique des tr`es petits grains et, `apartirdesraies ioniques visibles entre 5 et 40 µm([NeII], [NeIII], [SIII], [SIV], [ArII], [ArIII]), d’obtenir une estimation de l’intensit´e et de la duret´e du rayonnement ultraviolet. Par ce moyen, nous obtiendrions une vue plus pr´ecise des effets du rayonnement sur l’excitation des bandes aro- matiques et des petits grains carbon´es, dans des environnements diff´erant par leur densit´e, leur activit´e stellaire, leur m´etallicit´e, et l’´etat dynamique du milieu interstellaire.

—der´ealiser en compl´ement une synth`ese de populations stellaires des centres galactiques choisis, en tenant compte correctement des effets d’extinction et de m´etallicit´e, de mani`ere `a v´erifier notre r´esultat pr´eliminaire montrant la d´ependance des couleurs F15/F7 sur l’histoire de la formation d’´etoiles. 178 179

Annexe A Traitement des donn´ees et photom´etrie

Les diff´erentes ´etapes du traitement des donn´ees, d´ecrites dans l’Atlas, sont reprises ici et illustr´ees par des exemples, pour les observations en filtres larges. Les observations ont ´et´e effectu´ees avec le d´etecteur LW de la cam´era ISOCAM, qui est un photo-conducteur de 32 × 32 pixels op´erant entre 4 et 18 µm. Les artefacts instrumentaux sont en grande partie similaires `a ceux rencontr´es avec les CCD optiques, les effets de m´emoire except´es. En effet, le d´etecteur est refroidi par de l’h´elium liquide, aux fins de r´eduire le bruit thermique,a ` une temp´erature de 3a4K,` ` a laquelle les porteurs de charge ont une mobilit´etr`es r´eduite. Par cons´equent, la stabilisation de la r´eponse du d´etecteur est lente, et n’est en pratique jamais atteinte, parce que cela aurait n´ecessit´e des temps d’observation d´eraisonnables.

Construction des cubes de donn´ees :

Dans notre cas, o`u chaque galaxie a ´et´eobserv´ee dans les deux filtres LW3 (12-18 µm) et LW2 (5-8.5 µm), une observation est en g´en´eral compos´ee de : – quelques poses d’initialisation durant lesquelles les commandes de changement de configuration (filtre, temps d’exposition, gain, lentille) sont ex´ecut´ees. – plusieurs point´es, chacun comprenant de 15a ` 60 poses pour les galaxies de l’´echantillon de l’Atlas, dans le filtre LW3. –lesmˆemes point´es dans le filtre LW2. Pour la correction des effets de m´emoire, le plus grand nombre possible de poses doit ˆetre utilis´e, y compris celles ne contenant pas d’information photom´etrique utile (poses de changement de configuration et de d´eplacement sur le ciel). Trois cubes de donn´ees ont donc ´et´eg´en´er´es pour chaque galaxie, correspondant aux trois parties ci-dessus, en utilisant un outil du logiciel CIA (Cam Interactive Analysis) qui permet de d´ecouper une observation selon divers crit`eres. A l’int´erieur de chaque cube, les poses ont ´et´eregroup´ees par configurations identiques (mais incluant des poses utiles et des poses durant lesquelles le t´elescope se d´eplace vers la position demand´ee). Un vecteur binaire inclus dans les donn´ees brutes permet de savoir si le t´elescope est fixe ou se d´eplace.

Mesure du bruit de lecture et de photons :

Ce bruit a ´et´eestim´e pour chaque pixel, et suppos´e constant le long de l’observation dans 180 chaque filtre. Une carte de bruit est produite de la mani`ere suivante : - L’historique de chaque pixel est d’abord filtr´e par une m´ediane glissante sur 5 poses, de mani`ere `a en extraire les variations de haute fr´equence. - L’histogramme de ce bruit est calcul´e. A cause de la digitalisation, le plus petit intervalle possible est l’unit´e (en ADU : analog to digital unit). -Onajustesimultan´ement le nombre de z´eros artificiellement introduits par la m´ediane et la digitalisation, qui sont soustraits de l’histogramme et modifient sa normalisation, et la dispersion de la gaussienne la plus proche de l’histogramme. - Cette dispersion, divis´ee par le gain et le temps d’int´egration par pose est une estimation du bruit (σbruit). La figure A.1 montre un exemple d’ajustement.

Figure A.1: Historique d’un pixel (en clair) et filtrage m´edian (en noir), qui r´esulte dans l’histogramme de bruit montr´e`adroite.

Soustraction du courant d’obscurit´e:

Les variations du courant d’obscurit´eenfonctiondutemps,etpourdiff´erents temps d’int´egra- tion par pose, ont ´et´emod´elis´ees par Biviano, Sauvage, Gallais et al. (1998, “The ISOCAM dark current calibration report”), pour chaque pixel du d´etecteur, comme la superposition d’une d´erive en fonction du nombre de r´evolutions du t´elescope (sur une ´echelle de temps de plusieurs jours) et d’une autre d´erive au cours de chaque r´evolution (sur une ´echelle de temps de quelques heures). Les lignes paires et impaires du d´etecteur ont des comportements diff´erents, comme montr´e en figure A.2. Une variation a ´egalement ´et´etrouv´ee en fonction de la temp´erature du d´etecteur, mais n’est pas prise en compte dans le mod`ele (elle est n´egligeable en regard des deux autres effets).

En mˆeme temps que le courant d’obscurit´e est soustrait, les intensit´es initiales, en ADU, sont divis´ees par le temps d’int´egration par pose et le gain. A. Traitement des donn´ees et photom´etrie 181

Figure A.2: Trame du courant d’obscurit´e (`agauche)et image obtenue apr`es soustraction du mod`ele (`adroite), sur un exemple choisi pour l’absence d’impact de rayon cosmique. La 24eme colonne est vide parce que d´econnect´ee du circuit de lecture du d´etecteur. Elle a ´et´e syst´ematiquement masqu´ee, ainsi que les bords du d´etecteur lorsqu’ils ne sont pas suffisamment illumin´es (surtout en cas d’utilisation de la lentille donnant des pixels de 6).

Correction des impacts de rayons cosmiques :

A cause des temps d’int´egration du signal relativement longs (2 ou 5 s selon les galaxies), plusieurs impacts de rayons cosmiques affectent en g´en´eral chaque pose. Le cas montr´een figure A.2, sans impact, est plutˆot exceptionnel. De plus, les effets de m´emoire peuvent rendre l’´evacuation des charges cr´e´ees tr`es lente. La figure A.3 en montre un exemple. La d´etection des rayons cosmiques a ´et´e faite pixel par pixel, en appliquant un seuil de 4 σbruit `a l’historique filtr´e par une m´ediane glissante sur 5 poses.

Lorsqu’une source non uniforme est observ´ee, le bruit inclut des fluctuations de grande am- plitude qui ne sont pas attribuables au bruit de photons (qui varie comme la racine carr´ee du signal), mais qui varient de fa¸con approximativement lin´eaire avec le signal ; elles sont dues aufaitquelet´elescope oscille autour de sa direction de pointage, ce qui fait que l’image de la source se d´eplace sur le d´etecteur. Les fluctuations lin´eaires ont ´et´e suppos´ees affecter aussi les sources uniformes, pour rendre compte d’un bruita ` basse fr´equence mentionn´e dans le manuel d’ISOCAM. La valeur de σbruit a´et´emodifi´ee en cons´equence pour chaque point´e (uniquement pour la d´etection des rayons cosmiques, et non pas pour le calcul du bruit), de la mani`ere suivante :

1+αS2 σ2 = σ2 × pour S>b, (A.1) bruit mod bruit 1+αb2

(en n´egligeant le bruit de photons) o`u S est le signal, b le fond et α =0.01 a ´et´echoisiem- 182

Figure A.3: Exemple de 4 poses successives avec des traces persistantes de rayons cosmiques. piriquement pour reproduire les fluctuations observ´ees. Enfin, pour les sources les plus brillantes (S>b+ 100 ADU.gain−1), pour lesquelles les gradients de flux sont les plus forts et donc les effets des oscillations du t´elescope les plus importants, la valeur minimale du bruit a ´et´efix´ee `a 5% du signal (soit 4σ = 20%), de mani`ere `anepasd´etecter abusivement de rayons cosmiques.

Comme expliqu´e dans l’Atlas, les fausses d´etections aux changements de direction de pointage, lorsqu’on passe d’un niveau de flux faiblea ` un niveau fort et vice-versa, ont ´et´echerch´ees et annul´ees. La correction a ´egalement ´et´eit´er´ee dans un petit intervalle temporel autour des impacts d´ej`ad´etect´es, pour supprimer les r´esidus (en particulier les plateaux entre deux impacts qui se sont produits dans un court intervalle de temps). Les effets de m´emoires post´erieurs `a certains rayons cosmiques sont filtr´es ult´erieurement.

Correction des transitoires courts :

Les transitoires courts d´esignent la lente stabilisation de la r´eponse du d´etecteur apr`es un changement du niveau de flux. Avant fin 1998, seules des descriptions empiriques de cette r´eponse existaient. La principale, celle d’Abergel et al. (1998, http://www.iso.vilspa.esa.es/users/ expl lib/CAM list.html), que j’ai d’abord utilis´ee, suppose que suitea ` une marche de flux, la r´eponse instantan´ee est de 60% de la diff´erence de flux, et qu’elle est suivie de deux expo- nentielles dont les constantes de temps sont inversement proportionnelles au flux initial et au flux final. Par cons´equent, la stabilisation est plus rapidea ` flux ´elev´e. Comme toute l’histoire pr´ec´edente doit ˆetre prise en compte dans le calcul de la valeur stabilis´ee du flux pour chaque pose, la correction prend beaucoup de temps.

Un mod`ele reposant sur les caract´eristiques du d´etecteur, plus fiable pour les champs ne pr´esentant pas de forts gradients d’intensit´e, est ensuite apparu (Coulais & Abergel 2000), et j’ai retrait´e les donn´ees en utilisant ce mod`ele. Il est ´egalement non lin´eaire, mais d’application beaucoup plus simple, car il inclut une simple r´ecursivit´e d’ordre 1. Ce mod`ele a deux param`etres pour chaque pixel du d´etecteur, qui ont ´et´ed´etermin´es seulement pour des sources relativement faibles et uniformes. La correction de transitoires amplifie le bruit par un facteur de l’ordre de deux, qui a ´et´e pris en compte. Pour les sources brillantes et `a fort gradient de flux comme les centres de galaxies, la correction obtenue n’est pas meilleure qu’en appliquant la m´ethode d’Abergel et al. (1998), comme illustr´e en Fig. A.4. A. Traitement des donn´ees et photom´etrie 183

J’ai seulement apport´e des modifications minimes `a l’algorithme. Premi`erement, au lieu d’interpoler le flux des poses affect´ees par des rayons cosmiques, ces poses sont masqu´ees et la correction appliqu´ee comme si le flux de la pose suivante (en ADU.s−1)avait´et´eint´egr´e pendant la somme de son temps d’int´egration et de ceux des poses pr´ec´edentes masqu´ees, ceci pour ´eviter d’introduire du bruit, qui est amplifi´e par la correction de transitoires. Par ailleurs, au moment o`u le programme a ´et´e distribu´e, les m´ethodes d’inversion du mod`ele propos´ees n’´etaient pas applicables `a tous les cas ou bien ne convergeaient pas toujours : j’ai utilis´e`a la place une simple r´esolution par dichotomie qui converge toujours et s’ex´ecute rapidement.

Toutes les poses disponibles dans l’observation, y compris les poses de changement de con-

Figure A.4: Exemples de r´eponses avant (en clair) et apr`es (en noir) correction des transitoires courts. Les poses affect´ees par un rayon cosmique ont ´et´emasqu´ees et misesaunevaleur ` n´egative. En haut : correction r´eussie sur une zone d’illumination peu contrast´ee ; en basa ` gauche : correction d´efectueuse sur une sourcea ` fort gradient de flux (NGC 5195, le compagnon de M 51) ; `adroite:somme de 7 × 7 pixels autour du pixel pr´ec´edent, montrant que sur une zone ´etendue, la somme des corrections individuelles donne un r´esultat raisonnable (except´epour les poses post´erieuresa ` l’observation de la source intense). 184

figuration, ont ´et´e utilis´ees. Les conditions initiales (c’est `a dire le niveau de flux stabilis´e de la premi`ere pose), ont ´et´eestim´ees en utilisant la derni`ere pose pr´ec´edant imm´ediatement l’observation de la galaxie proprement dite non affect´ee par un rayon cosmique, et suppos´ee stabilis´ee. Pour les spectres, il est pr´ef´erable d’utiliser une autre approche. En effet, les poses initiales sont prises avec le filtre large LW2, ce qui induit une marche de flux de tr`es grande amplitude avec les poses prises dans des filtres ´etroits, et par cons´equent des effets de m´emoire critiques dans la partie des plus grandes longueurs d’onde du spectre. Mais ces poses initiales sont soit inaccessibles, soit en tr`es petit nombre. La r´eponse du d´etecteur durant toutes les poses de la premi`ere longueur d’onde (pendant lesquelles le flux r´eel est constant) est alors ajust´ee par le mod`ele de Coulais & Abergel (2000) pour estimer le flux stabilis´e`a cette longueur d’onde ainsi que le flux stabilis´edesposespr´ec´edentes.

Correction de la d´erive lente :

Il existe aussi des variations lentes de la r´eponse du d´etecteur, avec des oscillations, sur une ´echelle de temps comparable `a la dur´ee d’une observation. Elles sont parfois n´egligeables, mais dans un cas comme celui montr´e en figure A.5, il est impossible de d´eterminer le niveau du fond avec une bonne pr´ecision si cette d´erive n’est pas corrig´ee.

Pour chaque position du d´etecteur sur le ciel, la diff´erence de niveau du fond avec une position de r´ef´erence (la derni`ere de l’observation ou bien une position d´ej`a corrig´ee) est estim´ee sur un ensemble commun de pixels hors de la source. Pour les positions sur le ciel o`u le fond occupe une partie insuffisante du d´etecteur, la correction a ´et´einterpol´ee. Quelques exemples d’oscillations

Figure A.5: A gauche est montr´ee la carte de NGC 4178a15 ` µm obtenue sans corriger la d´erive lente, eta ` droite en incluant cette correction (toutes choses ´egales par ailleurs). A. Traitement des donn´ees et photom´etrie 185

N4178 (LW3) N4192 (LW3) N4567-68 (LW3) ADU/s/gain ADU/s/gain ADU/s/gain

position sur le ciel position sur le ciel position sur le ciel N4178 (LW2) N4192 (LW2) N4567-68 (LW2) ADU/s/gain ADU/s/gain ADU/s/gain

position sur le ciel position sur le ciel position sur le ciel

N4569 (LW3) N4579 (LW3) N4633-34 (LW3) ADU/s/gain ADU/s/gain ADU/s/gain

position sur le ciel position sur le ciel position sur le ciel N4569 (LW2) N4579 (LW2) N4633-34 (LW2) ADU/s/gain ADU/s/gain ADU/s/gain

position sur le ciel position sur le ciel position sur le ciel Figure A.6: Exemples de d´erives de grande amplitude dans des observations o`ulefiltreLW2a ´et´eutilis´eimm´ediatement apr`es le filtre LW3. L’ordonn´ee repr´esente la diff´erence entre le niveau du fond dans la derni`ere position sur le ciel et le niveau du fond dans la position courante (noter les ´echelles diff´erentes pour LW3 et LW2).

de tr`es grande amplitude sont montr´es en figure A.6 (dans des observations de galaxies de Virgo, pour lesquelles il est crucial que le fond soit homog`ene, car elles sont en g´en´eral peu brillantes). 186

Filtrage temporel des effets de m´emoire non corrig´es :

Une fois que la d´erive du fond est corrig´ee, il devient plus facile d’identifier par une proc´edure automatique les effets de m´emoire r´esiduels. Ils n’affectent sans doute pas de fa¸con significative les r´esultats photom´etriques dans des r´egions ´etendues, mais leur suppression am´eliore l’aspect final de la carte. Ils ont ´et´echerch´es dans les poses suivant des marches de flux descendantes de plus de 16 σbruit (cette valeur a ´et´e arbitrairement choisie comme le seuil `a partir duquel la correction de transitoires courts peut ˆetre inexacte), ou suivant un impact de rayon cosmique plus intense que 32 σbruit. Les marches de flux montantes sont plus d´elicates `atraiter,carsurles sources brillantes `a fort gradient de flux, la r´eponse montre des fluctuations dues `a des petites oscillations du t´elescope, et le bruit est plus ´elev´e. Par ailleurs, c’est dans le cas des sources les plus brillantes que la correction de transitoires courts est le plus inefficace. Le seuil pour

Figure A.7: Exemples d’effets de m´emoire filtr´es par la proc´edure automatique. En clair est montr´ee la r´eponse apr`es soustraction du courant d’obscurit´e et avant correction des rayons cosmiques et des transitoires courts ; la r´eponse apr`es correction de la d´erive lente (qui dans les cas montr´es est n´egligeable) est trac´ee en noir ; les poses masqu´ees apr`es d´etection d’un effet de m´emoire sont indiqu´ees par des croix au-dessus de la courbe de r´eponse. A. Traitement des donn´ees et photom´etrie 187

chercher des effets de m´emoire apr`es une augmentation du signal a ´et´e choisi de 100 σbruit,de fa¸con empirique.

Des crit`eres simples de d´etection de ces effets de m´emoire ont ´et´ed´efinis par essais successifs. Le flux m´edian dans chaque tiers des poses, pour une mˆeme position sur le ciel, a d’abord ´et´e calcul´e, en ´eliminant les impacts de rayons cosmiques d´etect´es. S’il existe des diff´erences entre ces flux m´edians de plus de 10% du flux au-dessus du niveau du fond et de plus de 4 σbruit,alors les poses concern´ees sont masqu´ees. Les effets de m´emoire qui ont une grande constante de temps ne sont donc pas d´etect´es par cette proc´edure, mais sont identifi´es visuellement dans les images finales (de quelques images `a quelques dizaines d’images `a examiner, au lieu des centaines de poses trait´ees de fa¸con automatique). En effet, la redondance spatiale des observations permet de v´erifier dans les images finales si les structures visibles sont r´eelles (auquel cas elles apparais- sent plusieurs fois aux mˆemes coordonn´ees du ciel et sur des ensembles de pixels diff´erents) ou artificielles.

Les baisses de la r´eponse en-dessous du niveau du fond, qui suivent certains impacts de rayons cosmiques intenses, sont plus facilement d´etect´es que les autres effets de m´emoire, pourvu que la d´erive lente soit correctement corrig´ee. Quelques exemples d’artefacts filtr´es sont montr´es en figure A.7.

Division par la r´eponsea ` un champ uniforme :

Apr`es avoir moyenn´e les poses valides pour chaque position sur le ciel, l’´etape suivante est la correction des variations de la r´eponse de pixela ` pixel (vignetting ou flat-field response). Dans toutes les observations o`u les images contiennent suffisamment de fond du ciel, la r´eponse `aun champ uniforme a ´et´emesur´ee `a partir des donn´ees d’observation, et non pas des donn´ees de calibration, qui sont fournies avec chaque cube de donn´ees.

Pour cela, un masque, destin´e`as´electionner les pixels voyant le fond, est d´efini sur une carte provisoire o`usontprojet´ees toutes les images (une par position sur le ciel). Ce masque est ensuite “d´eprojet´e” sur les images et la r´eponse `a un champ uniforme calcul´ee comme expliqu´e dans l’Atlas. La figure A.8 montre le r´esultat dans un cas o`u les donn´ees de calibration sont clairement inad´equates.

Dans l’Atlas est aussi mentionn´elefaitquepourmesurerlar´eponse `a un champ uniforme, la galaxie et les autres sources doivent imp´erativement ˆetre masqu´ees, et cela est discut´esur l’exemple de NGC 986. La figure A.9 en est une illustration.

Projection des images :

Les images sont projet´ees sur une grille rectangulaire en utilisant un programme de CIA. La correction de la distorsion n’a pas ´et´e appliqu´ee, parce qu’elle n’´etait pas encore au point au moment o`u elle a ´et´e essay´ee. La strat´egie adopt´ee est de masquer les sources `afortgradient de flux lorsqu’elles tombent pr`es des bords du d´etecteur (l`ao`u la distorsion est la plus grande). Sinon, leurs diff´erentes images se projetteraient sur la carte finale `a des endroits l´eg`erement diff´erents, en donnant une image d´eform´ee. 188

Figure A.8: Exemple de diff´erence particuli`erement marqu´ee entre les r´esultats obtenus avec deux versions de la r´eponse `a un champ uniforme, l’une calcul´eea ` partir de l’observation elle- mˆeme (`agauche),l’autre`a partir des donn´ees de calibration (`adroite).Cesr´eponsesa ` un champ uniforme (en bas) sont malgr´e tout visuellement tr`es similaires et pr´esentent une structure caract´eristique toujours observ´ee.

Figure A.9: La r´eponse `a un champ uniforme montr´eea ` gauche est obtenue en n’utilisant que les pixels voyant le fond, et celle de droite est obtenue en calculant la m´ediane de toutes les valeurs prises par chaque pixel, sans appliquer aucun masque. Le noyau de la galaxie a laiss´e des empreintes imm´ediatement visibles. A. Traitement des donn´ees et photom´etrie 189

Photom´etrie :

Le niveau du fond est d’abord estim´e`a l’ordre z´ero en ajustant l’histogramme des pixels hors-source par une gaussienne, dont l’´ecart-type donne une incertitude sur le fond. Ensuite, le pixel central de la galaxie est choisi manuellement, et d’´eventuelles sources secondaires (des ´etoiles ou des galaxies compagnons) sont masqu´ees interactivement.

La m´ethode adopt´ee pour mesurer le flux total d’une galaxie est d’int´egrer le flux dans des ouvertures circulaires concentriques de rayons variant d’un pixel. Lorsque la galaxie est circonscrite par l’ouverture, le flux int´egr´e augmente lin´eairement avec le nombre de pixels si le fond est homog`ene, et la pente de cette partie lin´eaire de la courbe donne la valeur du fond par pixel (r´esiduel, puisque le fonda ` l’ordre z´ero a ´et´e soustrait).

Cependant, comme les bras des galaxies s’´etendent fr´equemment jusqu’`a des zones proches des bords de la carte, cette m´ethode ne serait pas applicable si les parties externes des galaxies n’´etaient pas provisoirement masqu´ees, de mani`ere `a obtenir le plus grand nombre possible d’ouvertures ne contenant que des pixels voyant le fond, dans la partie centrale de la carte. La figure A.10 illustre cette m´ethode photom´etrique.

Figure A.10: Principales ´etapes de la mesure des flux totaux. En hautagauche: ` calcul du niveau du fonda ` l’ordre z´ero ; en haut `adroite: calcul du fonda ` l’ordre un sur la partie lin´eaire de la courbe, apr`es exclusion des parties externes de la galaxie ; en bas : calcul du flux total apr`es restitution des parties de la galaxie masqu´ees. Les lignes verticales en pointill´es indiquent la rencontre de chaque bord de la carte par l’ouverture, et la ligne horizontale repr´esente le flux de la galaxie tronqu´ee obtenu `a l’avant-derni`ere ´etape. La courbe en tirets indique l’estimation de l’erreur due aux effets de m´emoire. Dans le cas montr´e ici, le fond est ajust´esurx ∈ [500; 2300] et le flux est int´egr´ejusqu’`a x  5200. 190

S´eparation en r´egions centrales et disque :

Le profil radial de brillance de surfacea7 ` µma´et´ed´etermin´e en calculant la moyenne du signal dans des anneaux elliptiques, pour tenir compte de l’inclinaison du disque sur le ciel. Le profil est sur-´echantillonn´e, la largeur des anneaux ´etant en g´en´eral de 0.5 pixel. Le centre est fix´e en ajustant le noyau par une gaussienne lorsque c’est possible (dans presque tous les cas), ou bien manuellement. Le niveau du fond calcul´e`al’´etape pr´ec´edente est soustrait et les brillances manipul´ees en ´echelle logarithmique.

Un ajustement de ce profil est alors r´ealis´e par la fonction suivantea ` quatre param`etres :

1 f(r)=ln(coef ) − (r/coef )2 pour r ≤ r 0 2 1 sep = ln(coef2) − r/coef 3 pour r>rsep    r 2/ × − / 2 × / . avec sep =coef1 coef 3 1+ 1 2(coef 3 coef 1) ln(coef2 coef 0) (A.2)

Les r´egions circumnucl´eaires sont repr´esent´ees par une gausienne, la partie interne du disque par une exponentielle, et les deux parties sont tronqu´ees au rayon o`u les fonctions les repr´esentant sont ´egales. L’ajustement ne porte pas sur la totalit´e du profil de brillance, dont sont exclus la partie externe du disque, les structures de grande extension azimutale comme les bras, et l’int´erieur des anneaux circumnucl´eaires, ainsi qu’illustr´e en figure A.11. Si des bras spiraux polluent le profil du disque interne, comme pour M 51, alors la partie la moins structur´ee du disque externe est utilis´ee pour l’ajustement.

Correction de la dilution :

La correction de la dilution des flux des r´egions circumnucl´eaires par l’´etalement de la r´eponse `a une source ponctuelle (PSF) a ´et´etent´ee de deux mani`eres. La plus simple est d’assimiler ces r´egions centrales `a une source ponctuelle, bien qu’elles soient toujours plus ´etendues que la PSF, et de corriger les flux mesur´es `a l’int´erieur de RCNR (rayon angulaire) par :  Fcorr = Fmes / PSF, (A.3) (r≤RCNR) la somme de la totalit´edelaPSF´etant ´egale `a1.

Il n’est pas assur´e que cette m´ethode soit applicablea ` des centres galactiques comme celui de NGC 1097, bien r´esolus et structur´es. Le principe de la seconde m´ethode est tr`es semblable `a celui de CLEAN utilis´e en astronomie radio pour remplacer le faisceau instrumental par un faisceau “propre”, d´ebarrass´e des lobes secondaires (H¨ogbom 1974). Elle consiste,achaque ` it´eration,a ` chercher le pixel le plus brillant de la carte des r´esidus (initialement ´egale `alacarte brute),aensoustrairelaPSFcentr´ ` ee sur ce pixel avec un gain de 5% (de sorte que l’algorithme converge), eta ` restituer dans le mˆeme pixel de la carte synth´etique (initialement nulle) le flux total retir´e. Le calcul est stopp´elorsquelacartedesr´esidus est partout inf´erieure `a0.1%dela carte initiale. A. Traitement des donn´ees et photom´etrie 191

Figure A.11: Exemples d’ajustements des profils de brillance de M 51 (galaxie non barr´ee), NGC 1672 (fortement barr´ee) et NGC 4498 (dont la r´egion centrale est `apeiner´esolue). Les deux composantes de l’ajustement sont repr´esent´ees en tirets gris. La ligne verticale donne le rayon des r´egions centrales, RCNR,indiqu´e sur les images par un cercle noir. Le compagnon de M 51 et le centre de NGC 1672 ont ´et´esatur´es pour rendre les bras visibles. 192

Figure A.12: Image brute des r´egions circumnucl´eaires de NGC 1097a7 ` µm (`agauche)et image corrig´ee de la dilution (`adroite),en´echelle logarithmique.

Cet algorithme est en fait appliqu´e`a une partie Cc de la carte totale Ct, centr´ee sur le noyau galactique, et pr´ealablement modifi´ee pour ´eviter les effets de bord : `a chaque pixel de la partie compl´ementaire Ct − Cc est associ´ee une PSF, dont les ailes s’´etendant sur Cc sont d’abord soustraites de la carte des r´esidus. L’algorithme converge toujours pour le filtre LW2, ainsi que pour le filtre LW3 avec des pixels de 6, mais pas en LW3 avec des pixels de 3, sans doutea ` cause du rapport signal sur bruit plus faible. Dans cette configuration, la carte a ´et´ed’abord r´e-´echantillonn´ee avec un pas de 6.

La carte synth´etique est ensuite r´e-´echantillonn´ee sur une grille fine. Une deuxi`eme proc´edure it´erative est lanc´ee, la carte des r´esidus initiale ´etant la carte synth´etique obtenue par la proc´edure pr´ec´edente, dont on a soustrait le fond. Les ´etapes sont les suivantes : - Le pixel le plus brillant de la carte des r´esidus est cherch´e. - Le centre de la source ponctuelle correspondante (en nombre fractionnaire de pixels) est ap- proch´e par le barycentre dans 3 × 3pixels. - Une gaussienne d´efinie sur la grille fine est ajust´ee sur les 3 × 3 pixels en minimisant la somme des diff´erences absolues entre la carte des r´esidus et la gaussienne projet´ee sur la grille grossi`ere. Son ´ecart-type est autoris´e`avarierentre1.5 et 2,defa¸cona ` ce que le profil obtenu reproduise approximativement la partie centrale des PSF, qui ont des ailes tr`es ´etendues, et son amplitude, la plus grande possible, est contrainte par le fait que la gaussienne doit rester inf´erieure en tout pixelalacartedesr´ ` esidus. - La gaussienne projet´ee sur la grille grossi`ere est soustraite de la carte des r´esidus, toujours avec un gain de 5%, et elle est remplac´ee dans la carte corrig´ee par sa projection sur la grille fine. Le calcul est stopp´e lorsque le contraste du pixel le plus brillant avec les pixels environnants devient trop faible ou quand l’amplitude de la gaussienne devient inf´erieure `a un certain seuil.

Cette correction est illustr´ee en figure A.12 pour l’anneau circumnucl´eaire de NGC 1097. A. Traitement des donn´ees et photom´etrie 193

Contributions des diff´erentes erreurs :

Les diff´erentes sources d’erreurs et leur estimation sont expliqu´ees dans l’Atlas. La figure A.13 montre seulement quelle fraction de l’erreur totale est attribu´ee au bruit de lecture et de photons, `a l’incertitude sur le niveau du fond et aux effets de m´emoire (dans cet ordre de gauche `adroite), `a 7 (en haut) eta15 ` µm(enbas).A7µm, les effets de m´emoire sont toujours dominants, et ils le sont aussi `a15µm aux flux les plus ´elev´es.

Figure A.13: Fraction de l’erreur totale due aux diff´erentes sources d’erreur, en fonction du flux (sans tenir compte de l’incertitude sur la calibration en densit´edeflux). 194 195

Annexe B Publication : Composante baryonique des amas de galaxies

Cet article d´ecoule du travail de stage de DEA, mais son ´ecriture et la mise `a jour des donn´ees et des calculs ont n´ecessit´e beaucoup de temps pendant la th`ese : c’est la raison pour laquelle il est inclus ici.

Le th`eme de cet article est la quantit´e et la distribution spatiale de la composante baryonique des amas et groupes de galaxies (c’est `a dire le gaz chaud intra-amas observable en rayons X et la composante stellaire des galaxies). Pour ce faire, un ´echantillon de 33 syst`emes couvrant une large gamme de temp´eratures du gaz a ´et´eanalys´e, `a partir de donn´ees de la litt´erature en rayons X et en optique, avec une information spatiale suffisante. Les buts principaux ´etaient d’une part de comparer les r´esultats obtenus par l’application de deux estimateurs de masse – le classique mod`ele d’´equilibre hydrostatique isotherme, et un mod`ele de distribution de la mati`ere noire d´eriv´e de simulations num´eriques – et d’autre part d’examiner de possibles variations dans la quantit´eetlar´epartition des baryons en fonction de la taille des amas, ce qui peut fournir des contraintes sur l’efficacit´e des processus non gravitationnels dans la formation et l’´evolution desamas(chocseteffetsder´etroaction de la formation d’´etoiles).

The baryon content of groups and clusters of galaxies H. Roussel, R. Sadat & A. Blanchard 2000, Astronomy & Astrophysics 361, 429 196 Astron. Astrophys. 361, 429–443 (2000) ASTRONOMY AND ASTROPHYSICS

The baryon content of groups and clusters of galaxies

H. Roussel1, R. Sadat2, and A. Blanchard2,3 1 DAPNIA/Service d’Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette cedex, France 2 Observatoire Midi-Pyren´ ees,´ LAT, CNRS, 14 av. Edouard Belin, 31400 Toulouse, France 3 Observatoire Astronomique, 11 rue de l’Universite,´ 67000 Strasbourg, France

Received 1 June 1999 / Accepted 20 July 2000

Abstract. We have analyzed the properties of a sample of 33 temperature relationship, do not play a dominant roleˆ in heating groups and clusters of galaxies for which both optical and X- the intra-cluster gas on the virial scale. ray data were available in the literature. This sample was built to examine the baryon content and to check for trends over a Key words: cosmology: observations – galaxies: clusters: gen- decade in temperature down to 1 keV. eral – X-rays: general We examine the relative contribution of galaxies and ICM to baryons in clusters through the gas-to-stellar mass ratio (Mgas/M∗). We find that the typical stellar contribution to the baryonic mass is between 5 and 20%, at the virial radius. The 1. Introduction ratio (Mgas/M∗) is found to be roughly independent of tem- Clusters of galaxies are fascinating objects because their obser- perature. Therefore, we do not confirm the trend of increasing vations can in principle allow one to constrain the parameters of gas-to-stellar mass ratio with increasing temperature as previ- the standard cosmological model. In particular, they are widely ously claimed. used as indicators of the mean matter density of the universe. We also determine the absolute values and the distribution of Galaxy clusters have been shown to harbour very large quanti- the baryon fraction with the density contrast δ with respect to the ties of dark matter since the pioneering work of Zwicky (1933), critical density. Virial masses are estimated from two different but its exact quantity, its spatial distribution and above all its mass estimators: one based on the isothermal hydrostatic equa- very nature are still awaiting answers. tion (IHE), the other based on scaling law models (SLM), the Clusters are the most massive objects for which both the calibration being taken from numerical simulations. Comparing luminous baryonic mass (consisting of the X-ray emitting intr- the two methods, we find that SLM lead to less dispersed baryon acluster gas and the visible part of galaxies) and the total grav- fractions over all density contrasts and that the derived mean ab- itating mass can be estimated. Most often, the assumption of solute values are significantly lower than IHE mean values: at isothermal hydrostatic equilibrium (IHE) of the intra-cluster δ = 500, the baryon fractions (gas fractions) are 11.5–13.4% gas within the dark matter potential well is adopted to derive (10.3–12%) and ∼ 20% (17%) respectively. We show that this the total mass of clusters from X-ray observations, although is not due to the uncertainties on the outer slope β of the gas many clusters exhibit obvious substructures, both in the galaxy density profile but is rather indicating that IHE masses are less distribution and in the X-ray emission morphology. reliable. Examining the shape of the baryon fraction profiles, we Beyond the classical M/L ratio, clusters are at the center of find that cluster baryon fractions estimated from SLM follow a new cosmological tests of the mean density, which are differ- scaling law. Moreover, we do not find any strong evidence of in- ent in spirit and which are more global. Partly because of this creasing baryon (gas) fraction with temperature: hotter clusters new perspective, general observational properties of clusters do not have a higher baryon fraction than colder ones, neither have been investigated in detail in recent years. These studies do we find the slope β to increase with temperature. were triggered by analytical arguments as well as numerical The absence of clear trends between fb and Mgas/M∗ with simulations which indicated that clusters might have similar temperature is consistent with the similarity of baryon fraction properties in their structure. A first means of determining the profiles and suggests that non-gravitational processes such as mean density from clusters is to use their abundance as well galaxy feedback, necessary to explain the observed luminosity– as their relative evolution with redshift (Oukbir & Blanchard 1992; Bartlett 1997). A further important property of clusters is that their baryon fraction fb is expected to be identical (White Send offprint requests to: A. Blanchard ([email protected]) et al. 1993), reflecting the universal baryonic content of the uni-
Tables 1 to 6 are only available in electronic form at the verse. As primordial nucleosynthesis calculations provide very CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via strong constraints on the value of the baryonic density param- http://cdsweb.u-strasbg.fr/Abstract.html eter Ωb, determining the baryonic fraction in galaxy clusters 430 H. Roussel et al.: The baryon content of groups and clusters of galaxies allows to derive the matter density parameter Ω0 =Ωb/fb. precise information on the galaxy spatial distribution and lu- This surmise, when applied to a set of clusters, leads to a high minosity function, on the X-ray temperature and on the gas −3/2 mean baryon fraction fb, of the order of 20 h50 % (David et density profile, in order to be able to build up the density and al. 1995, hereafter D95; White & Fabian 1995; Cirimele et al. mass profiles for galaxies, gas and dark matter. This allows to 1997; Evrard 1997). Consequently, the critical value Ω0 =1 compute properly the baryon fraction rather than only the gas is disfavored (as the primordial nucleosynthesis is indicative of fraction as is often done. This is especially important for low −2 Ωb =0.0776 h50 η10/5.3 ± 7%, one obtains Ω0 ∼ 0.4). White mass objects, in which the stellar component is generally be- et al. (1993) have reviewed this critical issue in the case of the lieved to be relatively more important. Our sample comprises Coma cluster. clusters with temperatures from 1 to 14 keV, and therefore al- Some caution is necessary though, since there exists an ap- lows us to investigate several interesting quantities beyond gas preciable dispersion in the range of published baryon fractions. and baryon fractions, like the mass to light ratio and the ratio This scatter may be due to intrinsic dispersion in baryon frac- of galaxy baryonic mass to gas mass (possibly providing im- tions of different clusters. If real, it is important to understand portant constraints on galaxy formation), over a wide range of the origin of such a scatter. However, Evrard (1997) did not temperatures. All the data used here come from the literature, find any convincing evidence for a significant variation in the with the exception of Abell 665, for which we have analysed baryon fraction from cluster to cluster. Such a result is in con- an archival ROSAT image to obtain the gas density profile. In trast with Loewenstein & Mushotzky (1996) and D95. These fact, this cluster has already been studied from Einstein data by latter authors, from their study of ROSAT PSPC observations two teams (Durret et al. 1994; Hughes & Tanaka 1992), finding of a sample of groups and clusters of galaxies, have found a in each case a surprisingly very high gas fraction (respectively correlation between the gas fraction and the gas temperature,  50% and 33%). We will see this cluster provides a striking breaking the simplest self-similar picture (the different conclu- example of the scatter in different mass determinations. sion of Evrard could be due to the limited range of temperatures The sample is presented in Sect. 2. The methods to compute he used). A possible explanation for such variations, if real, the various quantities for each cluster in the sample is presented could in principle be the development of a segregation between in Sect. 3 and the results are presented in Sect. 4. In Sect. 5, we baryons and dark matter occurring during the cluster collapse, examine the trend with temperature for several quantities. operating more efficiently in massive clusters. However, this In all the present study, we assumed a H0 = −1 −1 mechanism has been shown by White et al. (1993) to be in- 50 km s Mpc and q0 =0.5 cosmology. sufficient to significantly enhance the baryon fraction and it is therefore unlikely that such a phenomenon could lead to a sub- 2. The sample stantial scatter in baryon fractions. Another possibility is that in poor clusters and groups, a part of the gas has been swept away in We looked in the literature for objects studied thoroughly the shallow dark matter potential well by galactic winds, being enough to allow us to compute the baryonic mass in galaxies, thus less concentrated than in massive clusters. This scenario the mass in gas and in dark matter at any radius. This is quite would also be consistent with the claim that the gas to stel- not a refinement, since both the baryon fraction and the galaxy lar mass ratio increases monotonically with the temperature of to gas mass ratio can vary very rapidly with radius, as will be the cluster (David et al. 1990, hereafter D90). Finally, a further seen in the next section. We therefore needed detailed informa- possibility is that mass estimates are not accurate and that a sys- tion, which drastically reduced the possible number of objects tematic bias exists with temperature. In any case, D95 derived that could be included in the sample. When a same object was this correlation from a very reduced set of objects (7 clusters studied by several teams, we applied straightforward selection and 4 groups) and it would deserve further investigation based criteria: for spatial X-ray data, for instance, we systematically on a larger sample. prefer ROSATobservations, because of its improved spatial res- As a consequence, it was one of our aims to address these olution and sensitivity, whereas for X-ray temperatures, Ginga questions with improved statistics. Moreover, in the baryon and ASCA satellites were preferred to Einstein MPC, most tem- problem, the reliability of mass estimates is rather crucial and as- peratures of which come from the catalogue of David et al. sumptions such as equilibrium and isothermality may introduce (1993). Recently, it has been noted that cluster luminosities and systematic differences in the results that we wish to examine temperatures might change noticeably when the central cooling in detail. The validity of mass estimates has been questioned flow emission is removed (Markevitch 1998; Arnaud & Evrard by Balland & Blanchard (1997). We have therefore taken the 1999). It is not clear which temperatures are to be used (espe- opportunity of this study to perform a comparison between the cially when using a mass-temperature relationship derived from standard mass estimate based on the IHE β–model and an al- numerical simulations). In order to keep our sample as homoge- ternative method derived from scaling arguments and numeri- neous as possible, we did not use cooling flow-corrected temper- cal simulations including gas physics (see Sect. 3.3), hereafter atures which are not always available. Furthermore Markevitch called the scaling law model (SLM). (1998) found that temperatures corrected for central emission In this paper, we present an analysis of a sample of 26 galaxy are in the mean 3% larger, which will be of weak consequence clusters and 7 groups taken from the literature. We required that in our average quantities. However, our treatment of the uncer- optical data were available for our objects and searched for a tainties on temperatures leads to large error bars when a large H. Roussel et al.: The baryon content of groups and clusters of galaxies 431

ROSAT archival image and the calibration routines of Snowden et al. (1994). We found that the gas surface brightness profile is well fitted by a Hubble-King law, and the X-ray emission can be traced out to a very large radius. The background level, which has been fitted together with the other parameters, is estimated with comfortable confidence. Spherical symmetry was assumed to derive the surface brightness profile in 0.5 arcmin wide an- nuli, although the X-ray map shows significant departure from sphericity; however, the effect of ellipticity on derived masses is known to be negligible (Buote & Canizares 1996). The cen- tral electron volume density ne0 was computed by matching the theoretical count rate with the 0.547countss−1 collected within a 30arcmin radius (after subtraction of the background), which amounts to solving:

E Fig. 1. Surface brightness profile of the intracluster gas in A665, in Emax − −σ(E)NH 2 − 1 g(T,E) e kT e A(E) the ROSAT bands R4 to R7 (0.44 to 2keV). Points represented by ne0 αE DT 2 dE an empty circle have been excluded from the fit because at these radii Emin E some background or foreground X-ray sources appear in the map. The 2 −3β θmax θ dashed line is the fitted background level. 1+ θ2dθ 0 θc (−3β+ 1 ) 2 2 dispersion in measured temperatures exists (see Table 1), as for θmax θ instance in the presence of a strong cooling flow. =2πS0 1+ θdθ, (1) 0 θc In some cases, optical data may be very uncertain because of projection effects and magnitude limitations, especially for groups whose galaxy membership is sometimes tricky to estab- −17 lish. However, we tried to identify objects for which data are with αE =1.02 10 SI and the angular distance D  reasonably reliable and we derived mean dynamical quantities 812 Mpc, and where A(E) stands for the energy dependence of for this sub-sample as well. Finally, it must be emphasized that the transmission efficiency. the X-ray limiting radius at which baryon fractions are estimated The results of this analysis are the following (for the bands is a crucial parameter, since both the galactic mass derived from R4 to R7 of ROSAT): β =0.763±0.023, θc = (112±5)arcsec a King profile and the X-ray gas mass given by the Hubble-King (which corresponds to 0.44 Mpc at the distance of A665), −3 −3 model diverge respectively for  ≤ 1 and β ≤ 1 (the definition ne0 =(2.85 ± 0.38) 10 electrons cm and RX lim = is given in Sect. 3), requiring that they be truncated. It is also 10 arcmin (= 2.36Mpc) with a central surface brightness S0 = −2 important that the baryon fractions of different clusters be com- (3.53 ± 0.26) 10−2 counts s−1 arcmin and a background −2 puted at an equivalent scale in order to test the scaling hypothesis surface brightness b =(2.2 ± 0.6) 10−4 counts s−1 arcmin . and if statistical conclusions are to be brought out from them, We used the gas temperature and foreground absorbing hydro- i.e. that we use the radius containing the same overdensity, while gen column density given by Hughes & Tanaka (1992) from information is actually available only up to the X-ray limiting their Ginga analysis, together with the formula of Mewe et radius RX lim which primarily depends on the characteristics of al. (1986) for the Gaunt factor and that of Morrison & Mc- the observations (detector sensitivity, integration time...). Cammon (1983) for the interstellar absorption cross section. At 14 X-ray and optical data are summarised in Tables 1 and 2, RX lim, the inferred gas mass is Mgas =(2.46±0.76)10 M, using a Hubble constant h50 =1. Notes on clusters which which is similar to the values found by Durret et al. (1994) and required a special treatment due to an incompleteness of data Hughes & Tanaka (1992). The hydrostatic mass is Mhydro = 15 can be found at the end. Optical luminosities are given in the blue (1.60 ± 0.24) 10 M. Our mass estimate from NFW’s dark band. When the blue luminosity was not available, we used the matter profile, computed with the EMN normalization (see sec- 15 following colors, corresponding to standard values for elliptical tion below) is Mdark =(1.41±0.25)10 M, and the resulting 15 galaxies: B-V = 0.97, V-F = 0.76, r-F = 0.58 (Schneider et al. total mass is MSLM =(1.68±0.33)10 M. The baryon frac- 1983) andR=F(Lugger 1989). tion amounts to respectively (16.3 ± 7.5)% and (15.6 ± 6.4)%. Hence A665 is a quite ordinary rich cluster whose baryon frac- tion seems reasonable if compared to previous values. 2.1. The case of Abell 665 We have also compared our gas mass estimates for the whole This cluster is one for which large baryon fraction estimates sample with other published analyses and found good agreement have been published in the literature. As these are surprisingly while the main differences are on fb, coming from the estimation high, we have found interesting to re-analyse this cluster using a of total masses as will be discussed in Sect. 5. 432 H. Roussel et al.: The baryon content of groups and clusters of galaxies

3. Analysis methods When no parameters for the luminosity function were found in the literature, we adopted the standard ones (Schechter 1975): 3.1. The stellar mass profile ∗ α =1.25 and MV = −21.9+5logh50. The stellar matter content can be computed at any radius from As a few clusters observed in X-rays do not have any avail- the cluster center using the projected number density profile of able spatial galaxy distribution (or with too poor statistics), but galaxies, their luminosity function and a mass to light ratio for only either a luminosity profile or even several total luminosities the stellar population calibrated on the observation of nearby given at different radii, we then assumed a King profile and fitted galaxies. Most often, the density profile is fitted by the common the few points by the resulting integrated luminosity profile: ⎡ ⎛ ⎞ 1 King form: 2 2 ⎣ ⎝ R R ⎠ 2 − L(Llim) Mass estimation is certainly the most critical aspect of recent studies of the baryonic fraction in clusters. Clarifying this issue where N(>L)=N ∗ Γ(1 − α, L/L∗) is the total number of is one important aspect of this paper. We derived the gravita- galaxies brighter than L, Llim being the limiting luminosity of ∗ ∗ tional mass in two ways: the observations, and Ltot = N L Γ(2 − α). The stellar mass enclosed in a sphere of radius R can eventually be written as: • The hydrostatic isothermal β–model : First, we used the stan- ∗ R M∗ L Γ(2 − α) 2 dard IHE assumption which, using spherical symmetry, trans- M∗(R)= 4πr νgal(r)dr. (6) Llim L Γ(1 − α, L∗ ) 0 lates into the mass profile: H. Roussel et al.: The baryon content of groups and clusters of galaxies 433

k dln ρgas(r) (in terms of comoving radius) which was used here to compute Mtot(r)=− TX r Gµmp dln r r200, writing: −2 −1 −1 r500 3k r 4 3 2 ρ(r) = βTX r 1+ . (10) δ(r500) = 500 = πr500 4πr dr Gµmp rcX ρc 3 0 −3 5X The total mass thus depends linearly on both β and TX. Hence, = 180fX ln(1 + 5X) − (14) if the slope of the gas density is poorly determined (and this is 1+5X the case if the instrumental sensitivity is too low to achieve a with X = r500/r200, where f = 1739/1500 is a corrective fac- good signal to noise ratio in the outer parts of the cluster), it will tor to transform dark matter mass into total mass, so that δ(r200) have a drastic influence on the derived mass. This mass profile is really equal to 200. Solving this equation gives X =0.66. results in the density profile: The relationship at z =0between virial mass and temperature ⎡ can then be written as: −2 −1 1 r 2 ∝ ⎣ 3 ρtot(r) 2 3 1+ TX =4.73 M15(r200) keV. (15) r rcX ⎤ BN did provide the following constant of normalization: −2 −2 r 2 −2 1+ ⎦ (11) TX =3.84 M15(r200) 3 keV. (16) rcX This difference is quite significant: it does correspond to a virial and in a flat density at the cluster centre. The isothermality as- mass 40% higher. Using this normalization will obviously sig- sumption can raise doubt, since Markevitch et al. (1998) found nificantly change the inferred gas fraction. evidence for strong temperature gradients in clusters, which may lead to IHE mass estimates smaller by 30% (Markevitch 1998). 4. Results However, the reality of these gradients has recently been ques- tioned (Irwin et al. 1999; White 2000). 4.1. Distribution of the various components The results from our analysis for each cluster in our sample • The universal density profile: An alternative approach is to are summarised in Tables 3 to 6: in Tables 3 and 5 mass es- use the universal dark matter density profile of Navarro et al. timates at r200 are derived from the SLM with the TX – MV (1995, hereafter NFW) derived from their numerical simula- calibrations respectively given by EMN and BN (used to com- tions: pute r200). Tables 4 and 6 contain mean dynamical quantities 3 over the sample at three different overdensities, r200, r500 and ρdark(r) r200 r2000 = 1500 2 (12) . The same quantities are also given with mass estimates ρc r (5r + r200) from the IHE model. where r200 stands for the radius from the cluster center where the mean enclosed overdensity equals 200 (this is the virial ra- 4.1.1. The binding mass −1 dius) and ρc is the critical density. It varies as r near the centre, being thus much steeper than in the hydrostatic case; Mass profiles of the various components for a few clusters are NFW claim this behavior fits their high resolution simulations displayed in Fig. 2, together with mass ratio profiles (right side). better than a flat profile. Furthermore, contrary to the β–model , Fig. 3 shows baryon and gas fraction profiles for the whole sam- the dark matter density profile obtained by NFW is independent ple. Quantities are plotted against the mean enclosed contrast on the shape of the gas density distribution. This will introduce density, which is the natural variable in the scaling model. A a further difference. The normalization of the scaling laws en- clear feature arising from Fig. 2 concerns the different behaviors sures a relationship between temperature, virial radius and virial of hydrostatic masses and total masses deduced from NFW’s mass. Here, this normalization is taken from numerical simula- dark matter profile, normalised by the EMN TX – MV relation- tions. Different values have been published in the literature (see ship: NFW profiles are more centrally concentrated, as could for instance Evrard 1997; Evrard et al. 1996, EMN hereafter; be foreseen from Eqs. 11 and 12, a property which is in agree- Pen 1998; Bryan & Norman 1998, BN hereafter). Frenk et al. ment with the density profile of clusters inferred from lensing (Hammer 1991; Tyson et al. 1990). In the outer part, where the (1999) investigated the formation of the same cluster with vari- 4 ous hydrodynamical numerical simulations. They found a small contrast density is smaller than a few 10 , the shapes of the den- dispersion in the mass-temperature relationship: the rms scatter sity profiles are quite similar, although some difference in the σ is found to be of ≈ 5%, EMN and BN lying at the edges of amplitude exists. In fact, profiles calibrated from the EMN TX – the values found, representing a 4 σ difference. EMN provide a MV relationship tend to be systematically more massive than scaling law between r500 and TX: with the isothermal hydrostatic model, with a significant disper- sion. The last column of Tables 3 and 5 gives the ratio between 1 2 TX −1 − 1 masses computed with both methods. The mean of masses es- r500 =2.48 h (1 + z) 2 Mpc (13) 10 keV 50 timated by the IHE β–model is significantly smaller than SLM 434 H. Roussel et al.: The baryon content of groups and clusters of galaxies

Fig. 2. Mass and mass ratio profiles for a few objects. The meaning of line styles is as follows: Left panels: thick line: SLM mass; thin line: IHE mass; dashes: gas mass; dot-dashed line: stellar mass. Right panels: thick lines: baryon fraction (continuous), gas fraction (dashed) and stellar to total mass ratio (dot-dashed) in the SLM case (with EMN calibration); thin lines: same quantities for the IHE model; three-dots-dash: stellar to gas mass ratio. H. Roussel et al.: The baryon content of groups and clusters of galaxies 435

Fig. 3. Profiles of the baryon fraction and gas fraction as a function of mean overdensity for objects with the most reliable data. Left panels show these profiles in the case of the hydrostatic assumption and right panels for mass estimates derived from NFW’s dark matter profile, with EMN normalization. Groups (TX ≤ 2keV) are represented with dotted lines, cool clusters (TX ≤ 5keV) with dashed lines and hot clusters with continuous lines. The group with a very steeply rising baryon fraction in the IHE case has β =0.31. masses (at RX lim): MIHE/MSLM =0.80 ± 0.03 with EMN’s from δ  5000 to the outer parts of clusters, in the case of normalization, and MIHE/MSLM =0.67 ± 0.026 with BN’s total masses derived from SLM as well as that of hydrostatic normalization. Clearly, such a difference will translate into the masses. Thus, the widely spread assumption that light traces baryon fraction estimates. mass is confirmed, at least at r ≥ rc. The influence of the choice of de Vaucouleurs galaxy density profiles as compared to King profiles is also clearly highlighted. In fact, in the core, 4.1.2. The X-ray gas dark matter is normally much more concentrated than galaxies, Second, the distribution of gas is more spread out than that of but using a de Vaucouleurs distribution, it turns out that the dark matter, which results in steadily rising baryon fractions concentration factor is considerably lowered and even reversed with radius (Fig. 4), as was already pointed out by numerous in the case of hydrostatic masses. Mixing the two shapes of teams, among which Durret et al. (1994) and D95. NFW also galaxy distribution in our sample, the result is an intermediate recover this trend in their simulations. This fact makes the choice behavior. of the limiting radius an important matter. In particular, extrap- olating masses to the virial radius (which is reached by the gas 4.2. The baryon fraction emission in only five clusters among our sample) could be very unsafe, especially for cool clusters, the gas of the most extended We find that inside a same object, the gas and baryon fractions of our objects with TX ≤ 5keVbeing detected only out to increase from the center to outer shells (Fig. 3 and Fig. 4), re- δ = 500. flecting the fact that the distribution of gas is flatter than that of dark matter, a trend similar to what is found by D95. Secondly, an interesting feature can be noted from Fig. 3: the baryon frac- 4.1.3. Mass to light ratio tion profiles versus density contrast are remarkably similar and The derived mean mass to blue luminosity ratio is shown in seem to follow a regular behavior, consistent with a universal Fig. 4. As it can be seen, M/LB remains remarkably constant baryon fraction shape, even in the central part (although with 436 H. Roussel et al.: The baryon content of groups and clusters of galaxies

found for the emissivity profiles (Neumann & Arnaud 1999) and gas profiles (Vikhlinin et al. 1999). Thirdly, the comparison of the graphs of Fig. 3 shows that the baryon fraction fSLM estimated from the NFW profile nor- malized with the EMN TX –MV relationship is less dispersed at all contrast densities. This effect is asymmetric: the high baryon fractions fIHE found with the IHE method disappear. The fact that fSLM appears less dispersed has already been found by Evrard (1997) and Arnaud & Evrard (1999). However, our work indicates that this feature exists at any radius. We also plot in Fig. 5 the histograms of baryon fractions derived from both the IHE and SLM methods, at the virial radius r200 but also at r2000, chosen because each object of the sample is detected in X-rays at least out to δ ∼ 2000. The comparison of the two indeed provides evidence for SLM masses to lead to more tightened baryon fractions than hydrostatic masses. At the virial radius, we found that the intrinsic dispersion is 50% with the IHE and 20% with SLM. This bears an important consequence for the interpretation of mass estimates as well as the interpretation of the baryon fraction. Clearly the fact that the baryon fraction is less dispersed in the SLM at all radii shows that this mass es- timate is safer and that the IHE method provides less accurate mass estimates, even in the central region where hydrostatic equilibrium is expected to hold.

4.2.1. Stellar to gas mass ratio M∗/Mgas

Also shown in Fig. 4 is the mean M∗/Mgas ratio as a function of overdensity, slowly going down after the central part. The galaxy density is indeed steeper than that of gas, decreasing in r−3 with  =1instead of r−2 for a typical value of β =0.66, and the situation is even worse when a de Vaucouleurs profile is used for the galaxy distribution. Again, the latter contributes in a large amount to the steep decrease in the central regions, whereas there the ratio is flat with King profiles.

4.3. Numerical results Average numerical results are presented in Tables 4 and 6. It is found that the mean baryon fraction using the SLM with the TX – MV normalization of EMN is 13.4% and the gas fraction 11.5% at r500 to be compared with hydrostatic results: respec- tively 19.2 and 17.0%. As expected, the two methods of mass Fig. 4. Average profiles for all clusters (groups included) with the most estimation lead to different baryon (gas) fractions. This differ- reliable data. Top: baryon fraction (continuous lines) and gas fraction ence is not negligible (≈ 40%) and is mainly due as already (dashes) in the case of SLM mass estimates with the EMN normal- noted, to the difference between the IHE mass and the SLM ization (thick lines) and hydrostatic masses (thin lines). Middle: mass to luminosity ratio for the whole sample (continuous lines), for King mass. The IHE mass can be 50 to 60% lower with respect to the galaxy profiles only (dots) and for de Vaucouleurs profiles only (dash- SLM mass (this is the case, for instance, of the groups HCG 62, SLM dots), with the same convention as previously. Bottom: stellar mass to NGC 2300 and NGC 4261). This difference between fb and IHE gas mass ratio, with the same line styles as for M/LB. fb increases when using the TX – MV normalization of BN (the mean baryon fraction being then 11.5 and the mean gas fraction 10.3%). Cirimele et al. (1997) found fb =23% for their 12 clusters included in our sample (and 20% excluding a larger dispersion). This behavior appears more clearly when A76), instead of our result of 19% (and 16%) using their pa- one is using the SLM model. This result is consistent with the rameters and the same hydrostatic β-model and their limiting baryon fraction following a scaling law as it has been already radius (they choose a uniform RXlim =1.5Mpc) and of 13% H. Roussel et al.: The baryon content of groups and clusters of galaxies 437

Fig. 5. Histogram of baryon fractions at r2000 and r200, with IHE masses in grey and SLM masses in black. The object at fb =0.7 is NGC 4261, which has the lowest X-ray slope β =0.31. using the SLM method. The disagreement is due to the adopted value for groups at r2000 is in good agreement with the mean stellar mass to light ratio (M/LB = 10.7 instead of our 3.2 value  5 of Dell’Antonio et al. (1995) for 4 poor clusters, after value). From the results of D95, it comes out that their 7 clus- correcting for the different M∗/L they have used. ters also have a mean baryon fraction of fb ≈ 23%. Thus, this is a confirmation of the divergence between hydrostatic β–model mass estimates and SLM’s masses. From a sample of 26 clus- ters among which 7 hot and 3 cool clusters are in our sample, 5. Correlations of the baryon population properties Arnaud & Evrard (1999) have made a similar analysis and de- with temperature rived in the frame of simulation-calibrated virial masses a mean In order to properly understand the baryon fraction in clusters it gas fraction at δ = 500 of  14% in rough agreement with is necessary to understand what the relative contributions of the our value of 12%. If the comparison is restricted to hot cluster gas and stellar components are. Several previous studies found subsamples, the agreement is as good (they found 16% to be that the stellar component is more dominant in low temperature compared with our 14%), and also at r200. A somewhat higher systems, the lower gas content of small clusters being possibly gas fraction (fg ≈ 17%) has been obtained recently by Mohr due to feedback processes. Our sample, large and spanning a et al. (1999), as compared to ours, which is probably due to the wide range in temperature, allows us to study these questions difference in the normalization of the TX − MV relationship. in detail. Another output from the present study is the mean total mass to blue luminosity ratio M/LB  270 at r2000 (the hydrostatic 5.1. The Mgas/M∗ – TX correlation assumption leading to M/LB  200), groups and clusters of all temperatures put together. However, when looking in more In this section, we examine a possible correlation between the detail at the three classes of groups (with TX ≤ 2keV), cool X-ray gas temperature and the ratio of gas mass to stellar mass, ≤ clusters (TX 5keV) and hot clusters, M/LB (at r2000) goes Mgas/M∗, at various radii. A strong correlation has been pre- from 200 to 270 and 340 respectively, with similar statistics (7 viously found by D90: from the analysis of twelve groups and groups, 10 cool clusters and 8 hot clusters). Hence, we disagree clusters with temperatures ranging from 1 to 9keV, D90 found with D95 who claim that the mass to light ratio is roughly con- that this ratio varies by more than a factor of five from groups stant from groups to rich clusters (using the group NGC 5044 to rich clusters. An increase of Mgas/M∗ with cluster richness which also belongs to our sample with M/LB  160, 2 cool has also been reported by Arnaud et al. (1992). clusters and 4 hot clusters, 3 of which are also in common with This trend has been interpreted as due to the galaxy forma- ours). It is worth noting that 2 of the 3 clusters in common have tion being less efficient in hot clusters than in colder systems. alowM/LB in our analysis: 150 for A85 and 170 for A2063. D90 suggest that the scenario for structure formation of hierar- This conclusion holds whatever the limiting radius: there is a chical clustering, in which large structures form after little ones factor of 1.7, 1.9 and 1.9 respectively between groups and hot by successive mergers, is adequate to explain their result: in fact, clusters when examining M/LB out to r2000, r500 or r200. as mergers go on, the intra-cluster gas is progressively heated As to the mean gas to stellar mass ratio Mgas/M∗, its values by shocks to higher and higher temperatures, as the size of the are summarised in Tables 4 and 6. We have computed this quan- structures involved increases; the higher TX, the more difficult tity to estimate the stellar contribution to the baryon fraction and it becomes for the gas to collapse and to form new galaxies. to investigate any correlation with temperature, which will be Hence, after some time, further galaxy formation would be pre- discussed in the next section. Let us simply mention that our vented in hot clusters, producing an anti-bias. 438 H. Roussel et al.: The baryon content of groups and clusters of galaxies

Fig. 6. Gas to galaxy mass ratio versus X-ray temperature. Open circles are for groups, filled circles are for clusters, and crosses refer to poor quality optical or gas masses.

In Fig. 6 we have plotted Mgas/M∗ against temperature at are necessary to explain the LX − TX relationship (Cavaliere the radii r2000 and r200. As can been seen, mean figures for et al. 1997). groups, cool clusters and hot clusters seem to show the sequence In order to examine this issue, we plot in Fig. 7 the baryon observed by D90, but in a less pronounced way: we find that fraction versus the temperature at different radii: RXlim, r2000 cool clusters have Mgas/M∗ which is ∼ 3 times smaller than and r200. Error bars were estimated by considering uncertainties for hotter ones instead of a factor of ≥ 5 in D90. Moreover, this on the temperature, and also on metallicity for groups. Uncer- apparent sequence weakens when plotted at r200: Mgas/M∗ is tainties on X-ray emission are small and lead to tiny errors on the only twice smaller for groups than for hot clusters. gas mass in the observed range (R

Fig. 7. Baryon fractions in the sample as a function of X-ray temperature. Top: at RX lim. Middle: at r2000. Bottom: at r200. Left: hydrostatic masses. Right: SLM masses. Groups are shown as open circles and objects with poor quality temperature measurements (and therefore masses) as crosses. that no clear trend of increasing β with TX is found. Although We have also examined the way the baryon fraction varies smaller β are found at the cool side, this might be due to a with β (Fig. 8). The baryon fraction derived from the hydro- IHE larger dispersion in β for the smallest potentials. We note that static β model, fb , does not vary with β in an obvious way: our result is consistent with the recent analysis of Mohr et al. if anything it decreases with increasing β (while no obvious cor- (1999). relation is found with TX, see Fig. 7). Such a trend, if real, would 440 H. Roussel et al.: The baryon content of groups and clusters of galaxies

Fig. 8. The slope β derived from the best-fit of a β–model to X-ray images plotted versus the temperature. On the right panel is plotted the baryon fraction (at r200) in the IHE β–model (filled symbols) compared to the SLM (open symbols) as a function of β. be unexpected. Using the SLM mass estimates, the baryon frac- 6. Discussion and conclusions tion is much more constant and less dispersed, even at a fixed We have analysed a sample of 33 galaxy clusters and groups cov- β (for β ∼ 0.5–0.7 the dispersion on fb/fb is 0.23 with SLM ering a wide range of temperatures. For all clusters, X-ray and estimates while it is 0.31 with the IHE). The fact that fb is optical data were gathered from the literature (except for Abell constant with β again differs from what one would expect if 665 whose ROSATPSPC data have been reanalysed by us). This reheating would have a dominant roleˆ in redistributing the gas has allowed us to investigate the structure of the various bary- inside clusters. onic components of X-ray clusters. Mass estimates were derived from two different methods: first we have followed the stan- 5.4. Implications on mass estimates dard hydrostatic isothermal equation (IHE method), secondly we have estimated the virial mass and mass profile by using the As we have seen (Sect. 4.2) the baryon fraction estimated with universal dark matter profile of NFW in which the virial radius the SLM method is less dispersed than with the IHE method. is deduced from the scaling relation argument (SLM method), This effect has been noted previously (Evrard 1997) and has the normalization constant being taken from EMN and from been interpreted as due to the observational uncertainties in the BN. estimation of β. The mass estimates (at some radius R) can be We find that virial masses (i.e. masses enclosed inside a written as: fixed contrast density radius) are systematically and signifi- MSLM = a1TR (17) cantly lower when one is using the hydrostatic isothermal equa- tion. After this paper was submitted, we have been aware of and a recent similar study by Nevalainen et al. (2000) who found that taking into account temperature profiles exacerbates this ∼ MIHE a2βTR. (18) difference, as inferred masses are then smaller. Examination of the baryon fraction versus contrast density has shown that the The fact that baryon fractions estimated with MIHE are more baryon fraction is more dispersed using the IHE. We have shown dispersed can be understood just because of the extra dispersion that this is not due to uncertainties on the β measurement but introduced by β (EMN; Arnaud & Evrard 1999). For this to be rather reflects the fact that the IHE method does not provide as due to the sole errors in the measurement of β, it would imply reliable a mass estimate as the SLM, neither in the inner parts nor that the dispersion in the measurements dominates the intrin- in the outer regions. Moreover the tightening of f SLM profiles sic dispersion, resulting in a tight correlation between β and b supports the idea that baryon profiles in clusters do have a rather fb, which is not obvious from Fig. 8: most clusters have a β in regular structure, i.e. that gas distribution is nearly self-similar, the narrow range 0.5–0.7, and the sample restricted to this range which is consistent with the recent studies by Vikhlinin et al. shows a larger dispersion for the baryon fraction computed with (1999) and by Neumann & Arnaud (1999) who found evidence the IHE. Therefore, we conclude that the large dispersion ob- of regularity in gas density profiles. However, when plugging served in the baryon fractions estimated from the hydrostatic β their mean standard density profile into the hydrostatic equa- model is intrinsic to the method itself leading to less reliable tion, these last authors found a mean total mass profile which is mass estimates, rather than to the uncertainty on β measure- different from the NFW profile (their mass profile is lower than ments. the one derived from numerical simulations). They have used H. Roussel et al.: The baryon content of groups and clusters of galaxies 441 the hydrostatic isothermal equation to estimate their total mass baryon fraction and stellar content do not show strong system- which probably explains this discrepancy. atic differences with temperature. Our mean gas fraction at the virial radius r200, using the SLM, is found to be in the range 12.6–14.6% (for h =0.5), Notes on individual clusters in rough agreement with Arnaud and Evrard (1999), when the BN or EMN normalization is used. The mean baryonic fraction – A85: Optical data for this cluster are unsafe and extend only SLM is fb ≈ 13.7–16.0%. It is important to emphasize that our out to 900 kpc. We used the observations of Murphy (1984) analysis shows that a larger baryon fraction could be obtained but, as the fit with an unusual galaxy density profile he per- when the sole hydrostatic equation is used, but it is reasonable forms is rather poor (and suffers from an inconsistency be- to think that this is an overestimation due to the mass estimator tween H0 =50and H0 =60), we chose to replace it with itself. Our analysis is consistent with an intrinsic dispersion of a standard King profile. 20% in baryon fractions (but this could be due to some system- – A401: As Buote & Canizares (1996) do not give the central atics), which means that our mean baryon fraction is uncertain electron density, we computed it with our program in the by less than 0.01. same way as for Abell 665, since both objects were observed As the observed luminosity-temperature does not follow the with ROSAT PSPC, using the galactic hydrogen column simple scaling expected from self-similarity, it is likely that non- density of David et al. (1993) and the count rate inside a gravitational heating such as galactic winds and additional en- given radius provided by Ebeling et al. (1996). ergy input by Type II supernovae play an important roleˆ in the – A2029: The same as for A401 applies. physics of the X-ray gas and may result in inflating the gas dis- – A2163: Optical data for this cluster are unsafe. No galaxy tribution. Metzler & Evrard (1997) have studied this possibility distribution was available. We therefore fitted the integrated and found that this is achieved more easily in low temperature luminosity profile given in Squires et al. (1997), but it was clusters (shallow potential wells). The consequence of such an not corrected for background galaxies and it extends only effect is an increasing gas fraction outwards within a cluster to 1.3Mpcwhereas RX lim =4.6Mpc. and a decreasing gas fraction with decreasing temperature. This – AWM7: We used the list of galactic positions and magni- effect is expected to be more pronounced in groups and cold tudes within 1o of the central cD of Beers et al. (1984) to clusters. From our sample, we do not observe such a trend of build an integrated luminosity profile, corrected for incom- the baryon (gas) fraction with TX whatever the method we use, pleteness using their limiting magnitude and the standard suggesting that non-gravitational heating does not have a domi- Schechter luminosity function. The optical core radius was nant influence at the scale of the virial radius. In turn, we confirm imposed to be the same as the X-ray core radius, which gives that the baryon fraction apparently increases significantly from very similar results as excluding the three innermost galax- the center to outer parts of clusters. ies (otherwise, the fitted core radius is too small and in fact, Several previous studies have shown a clear trend of increas- the King form is not a good representation of the central ing β with TX, while we do not find such a clear trend. Neither parts of clusters). do we find a trend of fb with β, which is consistent with the – Hydra A: The same procedure as for A2163 was applied, 11 absence of an fb–TX correlation, although we find a slightly with the three points given by D90: LV/LV =8.210 at IHE 12 12 decreasing fb (derived from the IHE method) with increas- 0.5Mpc, 1.310 at 1Mpcand 1.910 at 2Mpc. ing β, but with a large dispersion. This result is important as – HCG 62: We derived an integrated number count profile it shows that the scatter in the baryon fractions derived from from the list of galactic positions of Zabludoff & Mulchaey the IHE method is probably not due to the sole errors in the β (1998) (using their velocity criteria to select true members) measurement, but is rather due to the IHE mass estimator itself. and fitted it with the function in Eq. 7, assigning to each Our sample does not show any evidence of the strong indi- galaxy the mean luminosity derived from the limiting mag- cation highlighted by D90 that in low temperature systems, a nitude of the observations and the standard Schechter lumi- larger fraction of baryons is present in the stellar component. nosity function. The number of galaxies contained in this Although a trend could be present in our sample, the data are “compact group” is much larger than usually assumed (45 certainly consistent with a stellar to gas mass ratio being con- members with mB < 17 instead of 4 in Hickson 1982). stant with temperature (or mass), a further argument that non- – HCG 94: Ebeling et al. (1995) claim this object has been gravitational processes are playing a minor roleˆ in the overall misclassified and, from its X-ray emission, looks more like distribution of gas in clusters. a poor cluster rather than a compact group. Only 7 galaxies Finally, it appears that the properties of X-ray clusters are are generally attributed to HCG 94 but we made use of the still difficult to quantify because of the lack of large homoge- indication of Ebeling et al. that 12 more galaxies are ob- neous samples of clusters for which both optical and X-ray data served within a 1Mpcradius and at mB ≤ 18, to which are available. Such a situation is likely to improve with Chan- we attribute a mean luminosity as for HCG 62. The fit by dra and XMM. Nevertheless, the sample we have studied reveals Eq. 7 we perform relies entirely on this point (fixing the that clusters show important differences in the detail of the struc- core radius at the X-ray value) since inclusion of the cen- ture of their baryonic content, but that their global properties, tral galaxies would lead to a physically unacceptable core radius). Therefore, optical data for this object are unsafe. 442 H. Roussel et al.: The baryon content of groups and clusters of galaxies

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Table Xray data Parameters refer to Eq n is the central electron densityTemp erature error bars of clusters are

e

computed from dierent reliable references taking into account or not a co oling ow as the maximum of two estimates the

disp ersion among the measures and the quadratic mean of the quoted uncertainties this pro cedure pro duces a large uncertainty

when there exists a p ossibil ity for a strong temp erature gradient They are given at a condence level multiplyi ng when

necessary errors by the errors of Fornax and RXJ given by I and PA which condence levels are not stated

are assumed to b e

Fornax and HCG have got a twocomp onent gas density prole the one in the second line corresp onding to the nucleus

When no limiting radius R is given we assumed R Mp c T keV a relationship calibrated on other clusters

X lim X lim X

name z T ref R r n ref

X X lim cX e

keV Mp c Mp c cm

A D CNT

A
MF P

A
W CNT

A
MF BC EVB

A Perseus
F CK

A
W MG

A
D this study

A Hydra I
T LM

A WJF CNT

A
W CNT

A Coma
D BHB

A
W CNT

A
W CNT

A
MF BC EVB

A
W CNT

A
W CNT

A
EAB EAB

A
W CNT

A
W SK

A
W HBN

A
W CNT

A
W CNT

A
W CNT

AWM
W NB

Hydra A A
W D

Fornax
I I

HCG
PB PB

EMW HCG EMW

NGC MD MD

NGC DM DM

NGC DM DM

NGC
D D

RXJ
PA PA

Table Optical data The rst part corresp onds to the galaxy spatial distribution and the second part to the luminosity

function Eq

TOP typ e of optical prole stands for a King form Eq for a de Vaucouleurs form Eq for an integrated

luminosity prole Eq and for a de Vaucouleurs pro jected luminosity density prole cf Eq

and galaxies Mp c L L L Mp c

B L B

r for and corresp onds to r

c v

for and corresp onds to

name TOP M ref r M ref

B lim

c

B

Mp c

A CNT CNT

A M OH

A CNT CNT

A Db Da

A KS

A MG MG

A Db Da

A BO

A CNT CNT

A CNT CNT

A KG

A CNT CNT

A CNT CNT

A Db OH

A CNT CNT

A CNT CNT

A SN

A CNT CNT

A Db Da

A Db OHJ

A CNT CNT

A CNT CNT

A CNT CNT

AWM B

Hydra A D

Fornax F FS

HCG ZM

HCG HKA EMW

NGC ZM

NGC G

NGC N

NGC FS

RXJ PA

Table Dynamical quantities for the whole sample at the limiting radius r and with the r T normalization of EMN

X

the redshift is taken into account in this relationship The scalings with the Hubble constantareM and M h

tot

M h and L h

gas

name r M M M f ML M M

tot gas b B IHE SLM

Mp c M M L

  B

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

AWM

Hydra A

Fornax

HCG

HCG

NGC

NCG

NGC

NGC

RXJ

Table Average dynamical quantities for the ob jects with the most reliable data details on those that have b een discarded

can b e found in the paragraph notes on individ ual clusters using the EMN normalization Values are given at the virial

radius r from SLM but also at two other ones r the radius within which EMN claim that the hydrostatic equilibri um

is universally reached r whichwe preferred to use b ecause this represents the maximal extent of Xray observations that

is valid for the whole sample includi ng groups Groups are dened by T keV and hot clusters by T keV

X X

SLM NFWs DM prole IHE hydrostatic equilibri um

f f M M M M ML f f M M ML

gas b gas tot B gas b tot B

M L M L

B

B

at r

all at r

at r

at r

groups at r

at r

at r

co ol clusters at r

at r

at r

hot clusters at r

at r

Table Same as Table but with the r T normalization of BN

X

name r M M M f ML M M

tot gas b B IHE SLM

Mp c M M L

B

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

AWM

Hydra A

Fornax

HCG

HCG

NGC

NCG

NGC

NGC

RXJ

Table Same as Table but using the BN normalization

SLM NFWs DM prole IHE hydrostatic equilibri um

f f M M M M ML f f M M ML

gas b gas tot B gas b tot B

M L M L

B

B

at r

all at r

at r

at r

groups at r

at r

at r

co ol clusters at r

at r

at r

hot clusters at r

at r 218 219

Annexe C Liste de publications

Publications dans des revuesacomit´ ` edelecture:

– “Age and metallicity effects on galaxian absorption lines” H. Roussel & R. Sadat 1998, Comptes Rendus de l’Acad´emie des Sciences 326, p. 671-677

– “The baryon content of groups and clusters of galaxies” H. Roussel, R. Sadat & A. Blanchard 2000, Astronomy & Astrophysics 361, p. 429-443

– “An atlas of mid-infrared dust emission in spiral galaxies” H. Roussel, L. Vigroux, A. Bosma, M. Sauvage, C. Bonoli, P. Gallais, T. Hawarden, J. Lequeux, S. Madden & P. Mazzei 2001, Astronomy & Astrophysics 369, 473-509

– “The impact of bars on the mid-infrared dust emission of spiral galaxies: global and circumnuclear properties” H. Roussel, M. Sauvage, L. Vigroux, A. Bosma, C. Bonoli, P. Gallais, T. Hawarden, S. Madden & P. Mazzei 2001, accept´e pour publication dans Astronomy & Astrophysics

– “The relationship between star formation rates and mid-infrared emission in galactic disks” H. Roussel, M. Sauvage, L. Vigroux & A. Bosma 2001, accept´e pour publication dans Astronomy & Astrophysics

Comptes-rendus de conf´erences :

– “Barred spirals observed in the mid-infrared” H. Roussel, L. Vigroux, M. Sauvage, A. Bosma, C. Bonoli, P. Gallais, T. Hawarden, S. Madden & P. Mazzei in The universe as seen by ISO, October 1998, Paris 1999, ESA Special Publications 427, p. 957-960

– “Dust emission in spiral disks and in central regions of barred galaxies” H. Roussel, L. Vigroux, M. Sauvage 220

in Building galaxies : from the primordial universe to the present,XIXth Moriond astro- physical meeting, March 1999, 1999, Fronti`eres Editions, p. 25-28

– “Dust emission in barred galaxies” H. Roussel, L. Vigroux, M. Sauvage, A. Bosma & C. Bonoli in Dynamics of galaxies : from the early universe to the present,XVth IAP meeting, July 1999 2000, ASP Conference Series 197, p. 71-72 221

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